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Imagine an evil dictator. He imprisons his enemies in a labor camp, but they never do any work! He doesn't want to torture them, or force them to do that since he is using all of his soldiers to build him a beautiful castle. Instead, he decides to create an incentive: every time you do a certain task (such as carrying 500 stones a total of one mile), you get a coin from an automated machine. If you get some huge number of coins, you earn freedom!

The only problem is that he needs a door to release exactly one. He can't afford to send a soldier to guard the door, so he needs a door that will allow exactly one person to go through when a token is deposited into a coin slot. This is going to be based off of medieval-style technology, so no complex scanners or anything electronic.

I have thought of:

  • A revolving door (But two people could both fit in)

  • A weight based door (Two thin or young people could get out)

  • A scanner which would check for people (Too high tech)

I've wondered about this for a while, just out of interest,

Radvylf Programs
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  • Comments are not for extended discussion; this conversation has been moved to chat. – L.Dutch Oct 02 '18 at 16:14
  • Might be a better system that you have to have people buddied up with the same weight category and some sort of balance board, they can open the door to get in but if the board unbalances they fall into a pit of despair. – Ryan Jeremiah Freeman Oct 02 '18 at 20:04
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    It's easy to make a revolving door where two people can not fit in. Similar devices have been used for a very long time - they were originally called called "turnstiles" but they can be made like full-height doors. Some pictures here: https://en.wikipedia.org/wiki/Turnstile – alephzero Oct 02 '18 at 21:08
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    The question us invalid. You are creating an incentive structure where a hierarchy of prisoners would form. Those at the top would tax your coins and use them to make their own comfortable existence, eventually resulting in a mirror of the outside with armies and lords. The thugs at the top would let just a few out to keep the incentives working, adjusting the freedom rate per economic conditions like a central bank. In the end your internal army would prevail over the external. – Sentinel Oct 02 '18 at 21:51
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    Um, isn't this a turnstile? Or, I guess, you could go more complicated with a mantrap-style setup, but what am I missing here? – HopelessN00b Oct 02 '18 at 22:03
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    @Sentinel while you're probably right about the social effect, I'm not sure that invalidates the question. Addressing that effect is probably a good idea, but people enact imperfect systems all the time and unintended consequences are everywhere. – Morgen Oct 03 '18 at 14:00
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    Have you considered how to feed the prisoners? It seems like a severe loophole. Feeding prisoners means that you either (1) invest a lot of workforce into providing food or that you (2) give them enough space and tools so that they can feed themselves. (1) is definitely not an option, if you have no single guard to spare, you have no peasant either. (2) is also not an option, as unguarded prisoners given that much will break free in no time. Did I miss something? Another option is of course to starve them, but that will hardly make them a useful workforce. – Frax Oct 03 '18 at 19:46
  • How does the automated coin machine know you are putting in coins? – PStag Oct 04 '18 at 09:53
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    This possibility of bought freedom brings another dimension to a gulag. Typically these degenerate into tyranny with the most brutal psychopaths running the joint and taking most of the food, shelter, etc - now these people will be taking all the coin while trying to never seem to be about to leave as the possibility immediately kicks off a pre-emptive struggle for succession and/or an attack to seize the stash of coins. This could be a fun world... to write about, anyway. – Grimm The Opiner Oct 05 '18 at 08:32
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    This doesn't sound like a very evil dictator. Allowing slaves to earn their freedom? Not wanting to torture them? Not using his soldiers like cattle? – trysis Oct 06 '18 at 14:46
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    Why are his soldiers being stone masons instead of oppressing the proletariat and killing his enemies? – RonJohn Oct 07 '18 at 15:08
  • Strange understanding of evil. "I have no time being evil!" – rus9384 Oct 07 '18 at 15:57
  • Have the prisoners vote (with a coin) to confirm that the coins are true and there is just one person in the gates. Limit to one gate opening per day to minimise systemic abuse. – KalleMP Oct 07 '18 at 18:41
  • @Sentinel Actually, that's what I was hoping for – Radvylf Programs Mar 13 '19 at 12:58

41 Answers41

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Put it another way:

Just let the prisoner out when he/she pays for his/her weight.

The prisoner opens the door, gets in, and closes the door. The door is automatically locked. The inner walls are all slimy/oily and slant inward at the top, so he cannot get a hold on them. There is a slot for coins. He/she puts one big coin for each pound and when the required number of coins have been inserted, a hidden lock clicks, and an exit door on other side unlocks for a minute and he/she can walk out.

If the required number of coins are not inserted in the slot within a minute, all inserted coins are eaten (lost) and the input door is unlocked. The prisoner can go back and work harder to try again next time.

This also prevents locking the box forever by a malicious prisoner. Until the input door is closed and locked, all coins inserted in slot are just eaten, so do not even try to block the input door from locking.

The output door is only unlocked for a short time, then closed and locked again. Until it is locked, the input door would not unlock. The exit path from the output door is oily too, so you are forced to slide away and cannot help others somehow block it. Even if you could, that would just keep prisoners in camp so no harm to the dictator and an occasional patrol can simply put the offender back in the camp - with the angry mob he/she blocked.

The output door slides up, has a sharp bottom, and is heavy - trying to block it with your body would result just to being guillotined into two pieces. The inner piece would get cleaned by other prisoners soon (nobody would like to pay for half of someone else's corpse), the outer piece will slide away.

Dictator gets a coin for each pound of prisoners body and cannot be cheated. Even if two or three prisoners came in, they have to pay for their cumulative weight, so no cheat (other than reducing the rounding-ups) possible.

(Well, just be sure, that prisoners do not have tools and material inside, so they cannot sabotage it or dig holes through or make ladders to climb over the wall of the camp. But it is not specific to this kind of exit; this is common sense in unwatched camps.)


I am an evil dictator. After all, the 1 minute time is really short for inserting all coins :) No, really, I am not used to count in pounds. Maybe the timeslot is more like 5 minutes, or 1 minute after last coin inserted, maybe there are 10 and 100 pounds coins or something like that ... just the prisoner is not allowed unlimited or too long time inside ...


@theRiley: Technically it does not ensure "just one person", but as the OP put his question, his "evil dictator" have no need for extracting "just one person":

  • After all, he does not kill or torture his captured enemies, but he wants to free them instead (so there must be some feeding for them and really good one and stable, to ensure, that everyone has enough strength to make a lot of hard work, if he wish - and in medieval environment, many of the prisoners would be poor peons, which would like stay in the prison forever, just for the food - as work is totally optional - it would be better for them, then be "freed" to everyday hard work and big chance for starvation on regular basis)
  • Also that means, that the dictators peons have to provide food for prisoners - for free
  • He even allows them to self-organise and left them totally unwatched in the "prison" (or more like reacreational camp for big part of them?)
  • Also he allows them to make weapons and tools at their will (at least at stone-age level)
  • Work is purely optional and food does not depend on that - there must be an abundance of food
  • Even if they want some more materials - there are already bones and skin available (the evil dictator usually rules more than one village - sacrificial lamb could be chosen from those, who the prisoners does not love, or even hate - who said, that I cannot kill a prisoner from another country - is he/she even human for me?)
  • He provides infinite source of stones for making more tools
  • Well, we can even try to step over the stone age, as we can use metals from the coins (yes expensive in work, but totally possible, if anybody can earn "some huge number of coins, for earn freedom")
  • He allows his "automatic machines" containing precious coins just free to attempts to rob it (with lot of stones conveniently at hand) (and I can make holes where I wish with bow, bone and sand)
  • Also prisoners can use the stones to make stairs (or just hill) against any wall and run away.
  • He push ALL his soldiers away to work on his castle, not even left the smaller peon to go to prison once a week just to press "confirmation button" to say, that only one - not two - prisoners are leaving on one pay.
  • The pay can be distributed between prisoners at their will (just exchanging coins for say services, lost bets (it is easy to make cards/dices/other gaming properties in such a camp) or just bullying and rocketeering)
  • He can (and is willing to) pay for automats evaluating his coins as not fakes
  • He do not care to rule the prisoners, watch them, put them "in line" or something like that, he even do not care, if he has ANY prisoner at all (everyone can work and go free), but if he have any, he makes sure, that the prisoner lives far better than the average person in that time.

So in the light of all it - why he needs to release just one prisoner at time? The only logical answer is his pride, as he does not want to be cheated and seen as a fool (he is not seen as evil by prisoners anyway - more as really a benevolent and fair ruler, who gives food and games for free and allows you to leave, if you want and do some tedious work fork for him).

So this answer does not concentrate on exact wording of question, but to deliver, what our fair ruler (calling himself evil dictator) really wants - make sure, that no prisoner can cheat its exit due exit gate, without paying "some huge number of coins".

This answer shows, how to make it automatic, fair, hard cheatable and without even one sentient guard/advanced technology/magic.

None of the other answers so far provided a sure solution for a uncheatable one-person only, and I am afraid, that in the given restrictions it is impossible.

So I conclude, that what the dictator needs are satisfied by this solution and that satisfying the evil dictator inside the restrictions on low technology and no guards was the core of the OP's question, even if it does not follow the text of question to the last letter, as it solves the problem hidden in the question.

Peter Mortensen
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gilhad
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    I like the idea of using a quantitative instead of qualitative approach. – Matthieu M. Oct 02 '18 at 12:23
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    "He puts one big coin for each pound and when the required number of coins have been inserted, a hidden lock clicks, and an exit door on other side unlocks for a minute and he can walk out. If the required number of coins are not inserted in the slot within a minute, all inserted coins are eaten (lost) and the input door is unlocked." How the hell is someone supposed to put 150-200 pounds worth of coins in a slot in one minute. – Schrodinger'sStat Oct 02 '18 at 13:31
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    I am evil dictator, after all :) No, really, I am not used to count in pounds. Maybe the timeslot is more like 5 minutes, or 1 minute after last coin inserted, maybe there are 10 and 100 pounds coins ... just prisoner is not allowed unlimited or too long time inside... – gilhad Oct 02 '18 at 14:25
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    As a side note, this also incentivises the prisoners to starve themselves so that they weigh less and therefore have to pay less. Just a little bonus suffering. :) – Ethan Field Oct 02 '18 at 15:23
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    Yes, also keeps the price of food low :) – gilhad Oct 02 '18 at 16:39
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    @EthanField Or they take the fast route and chop off limbs – Hagen von Eitzen Oct 02 '18 at 21:35
  • Yes, I take it in calculation too - that means, that I get few coins, but prisoner will die before reching help (it is medieval after all). Plus many times I get the work done and the Prisoner would die before even reaching exit . Either way I have got done a lot of work, killed the Prisoner and I am not even officially responsible for his death :) And consider the great amounth of self-inficted pain and the knowledge, that he did it for nothing, but his own death as nice bonus :) – gilhad Oct 02 '18 at 21:44
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    Don't round weight up but instead round down. Now they are punished for entering together if they add up to another pound. – James Khoury Oct 03 '18 at 04:20
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    @HagenvonEitzen Imagine working as hard as you can to make enough coins to escape without an arm, chopping it off only before entering the room and finding out you're one coin too low so you have to work with one arm now. – Hankrecords Oct 03 '18 at 09:28
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    This system is based on a simple set of scales. If you can pay for the bubble of a spirit level, or the bar of a scale to balance centrally, you're out. Ties in exactly with this theory, but I just wanted to shout my love for the idea of paying for your weight. Brings new meaning to "Worth your weight in gold" (or more cuttingly, not-so). Also a circus/freakshow 'skill' was to guess your weight exactly, a skill brought to life and practiced by the peons under your rule, all vying to gauge an accuracy to the coins you'll need to accumulate, thereby building your world more. – Gray Roberts Oct 03 '18 at 10:49
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    This brings a new meaning to "cost an arm and a leg". – March Ho Oct 03 '18 at 14:17
  • how does this ensure that - exactly- one prisoner is let out ? i don't believe you answered the question at all ! – theRiley Oct 03 '18 at 14:23
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    @theRiley - I see what you mean, but the coin/weight system is an exact 1:1 ratio. If the coins don't match your weight, even if you have another man on your shoulders, you're calculating, pound-for-pound, the weight to get out. So although there's two men leaving, it's a valid transaction.

    Alternatively... there may be extra tech needed to make sure there is only one body going through at a time, although given the circumstance, is this necessary given the payment system in place?

    – Gray Roberts Oct 03 '18 at 15:57
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    @theRiley added edit to my answer – gilhad Oct 03 '18 at 21:40
  • You've really thought this through. – Strawberry Oct 04 '18 at 13:15
  • It seems like you could enforce the one-man-at-a-time rule a bit better with a couple of tweaks. First, and I think you may be doing this already, it's not clear - but require the weight to be exact to within some small fraction. So unless two people add up exactly and go over even a couple ounces, the door isn't opening. Second assign everyone in the prison a weight that they must maintain. Provide scales so they can track their weights and keep them where they are supposed to be. Assign weights so people's weight don't add up to each other, or add up to people they don't like. – Michael Oct 04 '18 at 17:32
  • It is not so easy, if you capture few hundreds of prisoners (and it is not too much). Also good fed noble can weight as much as 4-5 teenagers (say slingers), so it is easy to add to such weight. Body weight would inevitably grow for poor peons, given free food (so fat) and maybe work (add muscles), and as the price is "huge number of coins" the collection can take months to years, so teenagers would grow. Also body weight differ thru day about 1-3kg for eating, drinking and processing it. And do not forget, those free stones to increase weight preciselly. And no guards to enforce keep it. – gilhad Oct 04 '18 at 18:52
  • You can enforce the time limits spent in the airlock/counting chamber by having a coin fall into a reject slot every minute so putting the coins in promptly is a wise move. This will also incentivise single entry like the person mass round down system. – KalleMP Oct 07 '18 at 18:25
  • The system is also fair in a way, those who are big and burly must work harder, those who are overweight due to sloth would loose weight while working to their goal. Midgets, dwarves and children get discount. – KalleMP Oct 07 '18 at 18:27
  • Build the exit into a tunnel bored through solid rock of the cliff face separating the prison camp peninsular from the mainland (sharks and box jellyfish are farmed in the sea). – KalleMP Oct 07 '18 at 18:28
  • @Schrodinger'sStat Maybe not a slot. At least where I live, toll roads have automated booths where there's a big funnel on the side (so you can just throw in the coins from your car window) and they get counted automatically. – Suthek Oct 09 '18 at 10:13
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Just let 2 out if they manage to fit. The door only leads to the pit of despair, after all.

I mean you can't actually be freeing them. They might then be able to provide information to those on the outside about things that might compromise the prison's security.

So let 2 get out on occasion - the joke's just going to be on them.

GrandmasterB
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What about something like this? door

Allows the door to open just enough for 1 person to enter space and have to close the door to enter the room or outside. It could be very tight with spikes around the walls so they don't try and squeeze in.

clay
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    I'm almost certain that, if I was a slave, I would at least try a few times to squeeze in, EVEN if I have to be punctured a few times ! – Don Pablo Oct 02 '18 at 07:46
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    I could fit at least six children in there. Eight if I can break bones. – PStag Oct 02 '18 at 10:33
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    Just curious where that kind of door is used, I've never seen something like that before! – BruceWayne Oct 02 '18 at 14:09
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    @BruceWayne - http://www.cometaspa.com/en/Solutions/Unipass-system/ – Brian R Oct 02 '18 at 14:34
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    @BruceWayne They are used in many places where you have access cards and a high security standard. We have them at work in our datacenter, for example. – Philipp Oct 02 '18 at 14:53
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    And now I'm wondering how feasible climbing through the top of the door's gap would be. – Baldrickk Oct 02 '18 at 15:51
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    @Baldrickk If it was a problem, you could put a wedge-shaped plate right at the top (or just lower the ceiling) – chif-ii Oct 02 '18 at 17:45
  • @chif-ii that's an obvious solution - I was thinking more about that "secure" door in particular :) – Baldrickk Oct 03 '18 at 07:31
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    I don't think 'Tiny', the 20-stone giant, will fit. He'll be stuck in prison forever. – Pharap Oct 03 '18 at 13:14
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    @Baldrickk I suspect if you need to secure against the gymnastically inclined, you can pay double for the deluxe version w/ steel accordion-fold blocker panels – Nathan Smith Oct 04 '18 at 00:36
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    I orchestrated fitting 9 college men in a single phone booth (one of the red ones formerly common in London). Humans are surprisingly compact-able. – SRM Oct 05 '18 at 04:53
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A revolving or sliding door with a human-shaped "mold" will do the trick.

A problem with traditional turnstiles is that two people can easily squeeze into the space, by sandwiching up or sitting on another's shoulders. If you just make this space smaller, bigger people will not be able to pass through.

This problem can be somehow mitigated if a passing person would have to fit into a "mold" that is designed to fit one large human. It is much more difficult to pack two smaller humans into a seemingly sufficient volume if this volume is SHAPED.

Alexander
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Have you considered a psychological barrier?

It's medieval times, just tell all of the prisoners that there is a mechanism in the system - there are two doors joined by a hallway. You go into the first door, and it has to be closed and locked again before the second door unlocks. Tell everyone that there is a trap door (to a deep pit with spikes) set to open if there are two people in the hallway between the doors at the same time.

Any prisoner who has earned their freedom isn't going to want to risk death to let another person in/out with them.

Glen O
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    Perhaps make the mechanism actually work ... but controller by an unseen guard pulling a lever. After a live demonstration or two, the guard doesn't actually need to be at the station anymore for the threat to stick. – ThunderGuppy Oct 02 '18 at 20:08
  • @ThunderGuppy The OP specificially said they don't want to have to have a guard monitoring the system. – Pharap Oct 03 '18 at 13:17
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    @pharap Because of cost concerns, a permanent guard isn't viable, but I'm proposing one who's only there for a short while, just long enough to establish the illusion of the reliable pit trap. After that, they can be fired ... or killed, if that's your villainous speed. – ThunderGuppy Oct 03 '18 at 13:20
  • @ThunderGuppy Or chucked into the prison. Who's gonna believe there's no replacement guard? – Bob Oct 04 '18 at 03:03
  • Even better if the slaves inside can't actually check if the people who went in actually made it out. That way the threat is ever present – Miguel Bartelsman Oct 06 '18 at 12:25
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Human Trebuchet

Your an evil dictator, the safety of a freed slave shouldn't be too high a priority. Sling them into a nearby lake with a human trebuchet. Strap them to the device with a bracelet or manacle. If someone else decides to loosely attach themselves to the newly freed individual they will likely die on launch. If they strap themselves tightly they will drown once they hit the water.

XRF
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I think you may have over-constrained your poor evil dictator. A dictator who runs a labor camp without a single guard or employee in the camp is in a really strained position. Usually labor camps have dozens or even hundreds of employees, because you generally want to have an eye on these dangerous individuals. That's actually the point of labor camps. It's a way to keep your eye on people that you don't want to kill immediately.

As such, I don't think the dictator really need to have this precision. He doesn't need to worry so much if one or two people get out.

He doesn't have to worry because he makes the door as a revolving door. No windows like you'd see on a hotel revolving door. Just a solid blank wall that rotates. He puts a ratchet on the door to make sure it only revolves in one direction.

If you've seen the hotel revolving doors, there's a period where you can't go in and you cant go out. This is important because the purpose of these revolving doors is to keep air from rushing in from the outside, causing the hotel heater or air conditioner to work very hard.

Well, once you reach that point, you realize that the door is opening over a spike pit that will seal your doom. There was no freedom. This is an evil dictator, not an honorable master. If you try to stop and wait, so you don't fall into the pit, your friend who's behind you with his token will try to rotate the door and push you in.

As such, given the dictator can't even spare a single person to watch over this labor camp, I don't think he'll mind if 2 people die instead of 1. He clearly doesn't care all that much.

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    +1 without any guards no prisoners would ever work, and they would all escape by sundown (or as long as it takes to tunnel to freedom using their "work digging tools" in case they're underground) – Xen2050 Oct 02 '18 at 07:51
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    ...if they don't starve to death first – nzaman Oct 02 '18 at 12:00
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    +1, there must be at least one person running the camp, otherwise how do you feed the inmates, deal with medical problems, etc.? Surely there's a soldier or two who can't work on the castle due to sickness or injury. Just get that guy to do the releasing job too. – K. Morgan Oct 02 '18 at 15:21
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A particularly tight turnstile is the one person door you're looking for.

The coin verification system is pretty much the only thing that would be difficult for medieval technology, everything else is fairly simple: secure sections that can fit one person at a time, and gears that allow only enough rotation for one person to go through when the coin is inserted.

If it's good enough for amusement parks, it's good enough for prison.

Image of vertical turnstile

Giter
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  • In particular, you should be using a full height turnstile and ensure there is inward facing spikes or barbed wire over the top to prevent people from trying to climb over. The rest of the area should be fenced or walled off with a similar approach. – Shadowzee Oct 02 '18 at 02:23
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    A coin verification system can be built around clockworks - vending machines used mechanical coin sorters until the 1980's at least. Medieval clockworks were expensive, but robust. This clockwork monk is far more complex than a coin sorter: https://io9.gizmodo.com/5956937/this-450-year-old-clockwork-monk-is-fully-operational – pojo-guy Oct 02 '18 at 02:46
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    "If it's good enough for amusement parks, it's good enough for prison." A truism if ever I've heard one. – Jeremy Oct 02 '18 at 10:28
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    it's worth noting that usually these are installed on exits and rotate only one way - the fixed horizontal bars prevent access through the gate in the wrong direction. That said - I've crawled under these in the past. (a token operated one where I dropped my token which rolled through the gate - I went back through and deposited the token to gain entry again - well the token was worthless elsewhere, so no point in keeping it) – Baldrickk Oct 02 '18 at 15:54
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Another option is a kissing gate. These are designed so that you push the gate open, which is only a small travel distance. Then, there is a small area to stand. The gate closes behind you and you can walk out. Depending on the size of the standing area, only one person could fit. There is one at the University of Oxford that is like this.

Image from Google Image search

shlady
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  • What keeps you from holding the gate slightly open while walking past it to get out? – Kat Oct 02 '18 at 15:59
  • You can keep it from touching the other side as you walk out, but the space left is not wide enough for a person to get through. Think of the total width of the passage (travel distance of the gate) < thickness of 2 humans. – shlady Oct 02 '18 at 16:38
  • But if you kept it from latching shut, then someone can tailgate after you walk past it, right? – Kat Oct 02 '18 at 23:33
  • Oh, I see what you mean. Yes, that isn't addressed here. I have some half-baked thoughts on triggering a weight to move the door that is heavy enough that you couldn't overcome it/hold it open or something but nothing well-formed enough to add it. I'd welcome any ideas – shlady Oct 02 '18 at 23:47
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    Coincidentally I once had to climb over that exact gate when I found myself in that park after closing time. Those spikes aren't much of a deterrent if you're reasonably agile. – jymbob Oct 04 '18 at 15:23
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    @jymbob - Presumably Oxford thought razor wire wouldn't be a good look. I'm guessing an Evil Overlord would feel the opposite. – T.E.D. Oct 05 '18 at 12:09
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Water Clock as an airlock

The exit room is part of a water clock. The room is filled with water once per hour, and flushed every 12 hours. Inside the room is a seat with a helmet. If you have placed your coin(s) in the slot, the helmet descends upon whoever is in the seat and provides air for them for the next 12 hours. The floor drops (including the seat) to flush out the water. When the water is flushed out, so is the prisoner. Either they are flushed out dead because they didn't pay properly, or they are flushed out alive to freedom.

Bern Water Clock

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    I think suffocation is the only real solution. I looked at all these answers with the perspective of carrying a kid on your shoulders/back and most of them failed. If you submerged someone, and made the air-source un-removable—like a latch or kill switch—then you can't share the air and only one person can survive. – Nathan Goings Oct 02 '18 at 15:21
  • An air helmet? With medieval technology? Does it just have a really long bamboo straw or something? – Pharap Oct 03 '18 at 13:21
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    @Pharap A leather liner inside a metal cap, where the leather is sealed around the neck with a drawstring would allow air inside the helmet and the helmet to be sealed otherwise. A leather tube with a coiled wire inside would provide the air to be flowing through (both one in and one out). The token could trigger whether air or water flows through this tube. – Keeta - reinstate Monica Oct 03 '18 at 13:28
  • you would need a guard around once per hour. prison workers can't be trusted to do the job reliably :) – theRiley Oct 03 '18 at 14:34
  • @theRiley What job? The clock just runs from water flow. There is no need for any interaction other than the person attempting to leave. – Keeta - reinstate Monica Oct 03 '18 at 14:52
  • @Keeta - ah, my bad. very good then! – theRiley Oct 03 '18 at 15:14
  • Presumably the job of making sure the prisoner doesn't find some way to detach the leather tube or otherwise damage it so that air can come out somewhere other than inside the helmet. Although, the idea of requiring breathing underwater for 12 hours is intriguing and might prevent multiple breathers due to lack of sufficient air for that time, but it still might be tricky - provide just enough to barely remain conscious if not moving around, for instance, but some people are still going to require more air than others leaving room for a possible second person. – Michael Oct 04 '18 at 17:43
  • Genius. The only question left is how do you ensure someone doesn't just slash the helmet, making it impossible for anyone to escape afterwards. – ndnenkov Oct 05 '18 at 07:37
  • Actually given that we are constrained with medieval technology here, you can't have pressurised oxygen. So in order to have enough air for 12 hours, you will have something like a big balloon of air. Which prisoners can also make on their own, with say other prisoner's intestines/skin. – ndnenkov Oct 05 '18 at 07:56
  • @ndn I don't understand why you think there needs to be pressurized oxygen. Metal helmet fully covering all of head with leather attached to inside of it. Drawstring at the bottom to tie to neck, sealing it. One pipe in and one pipe out allows a flowing of air THROUGH that area. If the "toll" is paid, air flows through this helmet. If not, water does. In no way was OP asking about the impossibility of sabotage afterward, You can't build anything that is sabotage-proof. You can't build anything that can't be affected by persons on the outside. I think that point is out of scope. – Keeta - reinstate Monica Oct 05 '18 at 11:11
  • @Keeta, oh I thought that there was something in the helmet that contained oxygen, not that there is a pipe. – ndnenkov Oct 05 '18 at 11:14
  • Presumably you can sabotage-proof it by having the exit door exit into an area where the other prisoners can choose whether to open the outer exit door or drop you into the spike pit. So you don't want to upset them before you leave. – Ben Millwood Oct 06 '18 at 02:26
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One neck only

Although two thin people can try to squeeze into the same space, it would be essentially impossible for two necks to fit in a hole large enough for one neck. This is basically because human bodies are flat, but necks are round. The information I could find indicates that the vast majority of necks, from age 11 up, are between 10 and 16 cm in diameter. I reckon a 16cm neck could be squashed into a 14cm hole without causing strangulation, but two 10cm necks couldn't.

How do we utilise this? Well, we have a long underwater passage sealed at the top with a wooden cover. The passage is too long for a person to swim through without drowning. It is also very narrow so swimming would be quite awkward. However, the cover has a single neck-hole. One person can have his/her body submerged in the water and his/her head above water. Here is a simple way to implement this idea: (not to scale; the passage is too short)

The circle would need to be locked so that the cutout could not progress beyond the exit pool unless enough coins were provided, then it could be rotated to the entry pool. One person could enter the pool and place their neck in the cutout, then start edging sideways around the perimeter of the circle. Eventually the person would be able to pass their head through a cutout in the wall, much too small for a person above ground to squeeze through. Finally they would arrive at the exit pool, which would re-engage the locking mechanism, and the person would get out of the water.

The neck cutout would be too small for the person to pull their head beneath the water, thus preventing two people from sharing air during the journey.

Perhaps it would take an hour or two to push the heavy wooden circle all the way around, but since it floats on water, it is plausible for one person to be able to rotate it.

The wall does not need to be straight as depicted; for instance, it can surround only a small area within the "prison" side, which would be useful if a prisoner might need to quickly get out without being exposed to attacks by envious prisoners. It would still be a long way to the exit pool so someone trying to swim would not make it.

This doesn't have a built-in way of purging the dead. However, the floor could be made into a ledge, so any who drown would sink to a deeper area below. That said, whenever a prisoner leaves, the circle would rotate for a really long time so prisoners would realise they have no chance of holding their breath long enough.

This system is nice because apart from coin handling it doesn't have to somehow move and dispense items (like reusable keys or helmets) nor does it need any "active" process like filling or draining water. The locking mechanism would only need pegs, guides and simple latches.

Artelius
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  • this is brilliant. – zcaudate Oct 07 '18 at 10:07
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    I love this answer. That being said I think it would be possible to circumvent this with a tube, as you could fit a breathing tube along with the first prisoners neck. Then the second escapee would be able to breath through the tube and follow the first one who has their neck sticking out of the whole. Tubes could be made from something like leather. – trallgorm Nov 20 '18 at 21:13
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A Prisoner Guards The Door

A highly trusted and experienced prisoner is paid double the normal rate of freedom tokens to guard the door. The caveat is that if headcounts at the end of the day do not match the release manifest he will be severely punished and lose whatever freedom tokens he has accumulated. The door is a regular set of doors that are rigged via an iron bar so that only one may open at a time. The 2nd door is locked, and if the prisoner-guard is not presented the adequate number of tokens he will not open it and the offending person will lose all of their tokens. If two people try to leave they will both be credited with a negative balance equal to the amount it would take to buy freedom.

TCAT117
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    The method of having select prisoners guard their peers has been used successfully by prisons and POW camps for a very long time. This practice was one of the reasons for Milgram's Obedience Experiment. – Andrew Brēza Oct 02 '18 at 23:09
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    Hell, we STILL use it in an unofficial capacity. In prisons or jails "pod bosses" are afforded extra leeway and privledges if they agree to use thier power to maintain order and stability in thier prison or jail pod or block. Its an unspoken agreement between the guards and whatever inmate happens to be running things on the block. – TCAT117 Oct 03 '18 at 01:47
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    if you use a prisoner as a guard, you've just manufactured another guard. which goes against the fundamental problem requirement. – theRiley Oct 03 '18 at 14:51
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    nope. The problem is that he doesnt want to waste money HIRING a guard. You dont have to hire a guard if the guard is a slave paid in contrived non-monetary value tokens. – TCAT117 Oct 03 '18 at 19:49
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    You would still need to have someone trusted to verify counts at the end of the day, though. Which is not really any different than hiring one guard for your single exit door. – Miral Oct 09 '18 at 06:33
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Low air cavity

Think about this. The prisoner uses his money to open the door, goes through a corridor or whatever, until another door closes behind him. The point is that he finally finds himself in a closed room with little space. Even worse, the room starts getting filled with water. He probably thinks he's going to die. But the water doesn't reach the ceiling, leaving enough space for him to breath. At least while there's enough air (More people will consume more oxygen).

After some time, the exit door opens and the water is evacuated. If whoever is there is still alive, he's free.

scrp
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    different size people consume varying amounts of air over time - but it's an interesting concept. – theRiley Oct 03 '18 at 14:33
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    Also if they panic and start hyperventilating, but not as much as 2 people. And even with that possibility, the dictator needs to ask himself "do I really care if they nearly die? Or if some of them end up dying?". – scrp Oct 03 '18 at 15:12
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After spending tokens, the prisoner may pass through a revolving door. Let's say at most two prisoners make it through. Next, there is a jump from one ledge to another (and a gruesome pit beneath). The ledge to land on is a switch that automatically spins the other ledge by 180 degrees. As soon as the first person lands, the second person gets flushed into the pit.

This principle can be extended in case you believe 3 could make it through the revolving door and one may be able to throw the other two onto the ledge instead of one after the other trying to jump. After landing on the ledge (and flushing the third into the pit), the two face another ledge (or another problem of a similar structure). In other words, you don't need a barrier that lets exactly one prisoner out, you only need a sequence of problems that are more difficult the more prisoners try to jointly achieve it.

HRSE
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  • this is a decent and fairly low-tech answer. well done!. probably could do it in one stage by making the opposite ledge only big enough for one set of feet, and far enough that you couldn't carry someone with you on the jump reliably. or make them go back and forth a few times between the ledges, as you were alluding to in terms of a sequence of challenges. but - i dont see this is a guarantee - a spectacularly strong & athletic person might carry a buddy through. some other filter would probably be needed to drive it to certainty. you also need a power source for your rotating ledges ;) – theRiley Oct 03 '18 at 14:43
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Ignoring that the dictator really can afford one person to guard the door, and might want to have some sort of celebration when somebody actually gets out and require them to wait for the celebration....

Your original problem was that sometimes one person is as big as two others, or as heavy as two others, etc. So most of the ways you could measure whether it's one person or two would fail in the extreme case.

The easiest thing is to just ignore the problem. Occasionally let two tiny people get out in place of one. Or a great big person can't get out and let him complain about that.

But failing that, with medieval technology, have the exit through a moat, a water-filled corridor. Above There's an air gap at the top that a person can breathe through. To get out, he puts his neck in a stock that leaves his head in air and the rest of his body in the water. The only thing that can stick into the air is his head. Anybody who tries to swim through gets no air, and anybody who tries to crawl through the air space finds a hole at the end just big enough for a head. Two people, no matter how small, cannot fit both their necks in the stock. Arrange that it takes long enough to get through that somebody who tried to hold their breath and swim through the bottom must drown.

I think this could be done with medieval technology. The device to make it take a long time to get through might be tricky.

Would it work? Probably not. People are tricky, and if you leve them unsupervised they will think of something. A snorkel that can stick through the stock past somebody's neck. Etc.

If you make any part of it out of wood, people will find a way to saw it, cut it, or burn it. If it's metal bolted together, they will find a way to get pliers or wrenches. If the water is above the water table, they will find a way to siphon it out.

Anything one engineer can make, another engineer can unmake given sufficient unsupervised time. But it's fun to try to think of ways.

J Thomas
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The door leads to a long corridor full of poisonous gas; the token is an antidote tablet.

Not quite sure if it would be feasible with medieval technology, but you do have some artistic license. Let the gas be slightly corrosive so that any improvised gas mask or other type of protection will be inactive.

IMil
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They have security doors.

You have a hall with a door at both ends.

First they have to pay in their token(s).

When the first person enters the door closes behind them. After some kind of check, say a weight check, and after door 1 closes and locks then door 2 opens.

You can even combine this with a turnstile before door 1 to prevent 2 people from going through door 1 at the same time. Make turning the turnstile hard so they have to push it hard to turn it, this would make it even harder for 2 people to reach door 1 at the same time.

cybernard
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    And if there is a kid ridding piggyback of another kid, or a light adult ? How do you make sure the doors are really locked and not stuck ? – Don Pablo Oct 02 '18 at 07:58
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Your freedom costs your weight in gold!

This way, you can use your weighing system and be sure of the correct result. If people want to pool their resources then they'll still only be able to let out skinny prisoners who, as other answers have stated, have probably been working harder than the larger prisoners.

It makes it unfair on those with glandular conditions and whatnot but this is a medieval prison, fairness be damned!

Disgusting
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The 'door' is a submerged tunnel. Across the tunnel are air pockets that have only enough space to fit one person's head. Along the way there are turnstile doors that need a key(given from the automated machine) to open so the key cannot travel back down the tunnel.

*Since swimming was a rare skill a rope to pull yourself along would work just as well.

PStag
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  • I had a similar idea whereby the initial token unlocks some kind of pipe/reed that can be used as a snorkel, the prisoner has to travel through a room filled with water with a slot cut into the roof that's wide enough for the snorkel but doesn't allow enough space for someone to put their head above water and breath. The same problem applies to both, there's nothing to stop two people sharing the snorkel, or two people taking it in turns to pop their head into the airspace, take a lung-full and then make way for their accomplice. – delinear Oct 02 '18 at 11:12
  • @delinear, I also thought of a pipe/reed. I honestly think suffocation is the only true solution. Simply make the pipe/reed a one time connection. That is, it can only be used by one person, if they try to share it, it fails and they both die. – Nathan Goings Oct 02 '18 at 15:23
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    you'd need a guard to reset the keys – theRiley Oct 03 '18 at 14:45
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While this is a weird set-up, a single person "door" is possible.

Imagine a see-saw filling a hallway. The pivot point is on the middle of the hall, and looking at it from the starting point it appears to be a ramp. You walk up the ramp for a bit, and then it starts to level off. Walk past the pivot and it descends to ground level, and you walk out the exit.

If your buddy tries to run on the ramp behind you, the combined weight keeps the ramp locked in the "up" position. Once you are off the ramp, it can either be reset by a flunky, or the ramp is weighted enough on the "entry" end to swing back to the "up" position. The technology isn't difficult, an inverted roof truss arrangement works, much like this car ramp system:

enter image description here

One end is down to make a ramp, and in this case the other end is braced so the car remains level once the car rolls forward enough to tilt the ramp. Remove the brace and the car can roll forward and off

enter image description here

A different variation of the same idea

A few refinements could be to have the entry behind a door or right angle so only one person at a time can actually approach the ramp. So there is a potential one way door without too much high tech.

Thucydides
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  • Definitely possible with medieval tech, and seems like it would work well! – Radvylf Programs Oct 02 '18 at 01:22
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    that just makes the problem recursive the ramp only works if ony one person can approach it, how do you make sure only one person approaches it? – John Oct 02 '18 at 01:51
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    What John said is right... if two people hug each other and walk up the ramp, they would trick it into thinking there is one person. It works with a car, because a car is going to be long enough that you can both block the entrance while only allowing one, but with people, your ramp would either be too short to block a person, or long enough to allow multiple people to stand on it. The follow up refinement comment is just the same problem in a different location. – Shadowzee Oct 02 '18 at 02:13
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Option 1: a turnstile

I've worked in a few places where they had these:

A regular full-height turnstile

You can't really fit two people, but I guess desperate people might try it anyway. To counter that, the solution is to make it tighter. I'd assume those who have carried enough big rocks to earn freedom would be rather lean, so all you need is a turnstile where an average human can barely fit. Plus-size prisoners be damned, they clearly haven't done enough work. If you already have the medieval token validating machine, this is trivial.

Option 2: part-time guard duty

Sure you can't afford a full time guard for some reason. But then you could just decide people can only get out Saturdays from 9 to 10. They've waited this long for freedom, they can wait for next Saturday. Like the poet says, everybody's working for the weekend. Of course, there are problems to this, the possibility of bribery, letting someone out by mistake, or, worst, union strike.

Option 2, part 2: part-time you

Like the French say, you're never better served than by yourself. You are immune to bribery and mistakes, and besides you do want them to know you let them out because you're such a generous and nice guy. Never neglect the effect of the personal touch.

AmiralPatate
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  • "everybody's working for the weekend." Pretty sure that's talking about working so you have enough money to have fun on weekends. Not actually working ON the weekend. – Aethenosity Oct 02 '18 at 22:07
  • @Aethenosity What I meant was it's working so you can get out and have your freedom, so it totally applies. – AmiralPatate Oct 03 '18 at 05:50
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A saddle riding on a track that twists and turns like a bucking horse will force the person to hold on with both hands and any other person will have their center of gravity off center, and that would make the bucking movement very hard to counter, throwing the other person or both of them off the saddle into the try-again pillows (concrete).

workoverflow
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    This could be overcome with skill, luck, or two people tied together. – Nathan Goings Oct 02 '18 at 15:24
  • @NathanGoings then anything could be overcome with skill, luck or two people tied together, except for heartbeat scanners that have lasers that eliminates all but one beating heart. (then again it could be thwarted by drugs that slows your heart so you could appear dead) – workoverflow Oct 03 '18 at 06:47
  • not totally foolproof, but a cool idea, and almost foolproof. love those try-again pillows ;) – theRiley Oct 03 '18 at 14:58
  • @workoverflow, I think the water/suffocation related answers would be hard to beat considering the constraints to the original question. It's similar to your heart argument but lungs and more realistic. – Nathan Goings Oct 03 '18 at 20:24
2

How about a very long sluice / underwater tunnel?

You simply then make only one diving helmet available (and weights for feet etc), with a deadly ceiling (venomous spikes?). This way you can't have a floaty hanger-on (the spikes!); the helmet is typically so heavy you can't swap mid-swim (and would live on 50% of the expected oxygen while doing more effort due to costume swapping) --- taking helmet off you'd float up, putting it on underwater you'd plummet down out of your mate's reach. The pressure differences would get you, if you're not simply following the underwater-staircase... and it's dark in there too, no communication possible.

Combine that with a turnstyle (that looks like it will only let one person pass, but inevitably people will try two and succeed at some point) so there's no information feeding back to your prisoners (and don't re-imprison any successful leaver!). You'll need occasional cleanup to display the dead bodies of failed escapees.

Actually that may be the most straightforward solution: Turnstile followed by a bendy route-out-of-sight. Employ for a limited time guards that kill both leavers, then 48hrs later drop the dead bodies in the main prison... this will raise enough questions and make them stop trying: How!? What?!
Without this rule, when one has enough money to escape, then a cheapskate asks "can I tag along?"; you say "yea, gimme all your gold; but I take care of myself if in doubt out there" --- but in this situation it's "I'm paying full price I'm not taking any risk I go alone". I think it's cheaper to employ guards for a week at the begin than to construct elaborate defences... and maintaining defences is expensive too. YOu'll want a periodic headcount to see if people are escaping and you need to shore up defences/ deploy 'project fear' to stop them from attempting. [Maintenance costs may depend on climate too...]

The prisoners have no idea if the exit is guarded at all (if there's actually two turnstyles, otherwise the legitimate leaver might report back. You'll need some communication barrier otherwise the successful leaver will send messages back in with balloons, skywriters, carrier pigeons, rocks-with-messages, fireworks/signal arrows, ... .

user3445853
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The Centrifugal Sally Port Singleton Sortation System Solution

The system comes with a complete set of instructions posted both inside & outside the sally port, complete with clear illustrations, explaining the tasks required to complete the emancipation process.

Prisoners attempting the undertake the emancipation process do so entirely at their own risk.

The first sortation is by aptitude & a certain minimal level of mechanical competency. Those prisoners not capable of following instructions will have to adapt themselves to prison life indefinitely, or assume the various risks as described below.

The interior chamber features a centrifugal sortation mechanism. The interior door opens for a fixed interval upon inserted coin weight exceeding the price counter-weight, this easily modified as the price of freedom fluctuates with market forces. The prisoner enters.

The system is in its first operating state. It is driven by a mechanical stepper actuated by sisal twine connecting linkages. As each step is completed, actuators or timers trigger the stepper state advance step wise, until the emancipation cycle is completed, the door opening for a fixed interval, during which the freed prisoner exits, then closes. Each step has a timer-driven delay which, if exceeded, resets the chamber to its initial state, the interior door opens and the prisoner, bereft of his coin, returns to from whence he came, likely motivated in future to read the instructions more carefully.

First the prisoner ready to be freed, once inside, must wind all the timers, then charge all the springs, in the following order: carousel brake engage spring, inner then outer door spring assemblies, the centrifugal shaft main coil spring, hand and feet irons compression springs.

The irons are all formed to fit the normal range of adult human limbs - prisoners missing more than a single member, or having deformed or in any manner terribly oddly shaped limbs get extra rations, no freedom ever. All are briefed about the situation in their orientation session the first day, so no unhappy surprises later.

As soon as all timers have been wound & each spring is charged, the stepper advances, opening access to the carousel. The prisoner steps onto the carousel near the main shaft where there are positioned hand and feet irons.

Feet and hands are placed into the irons carefully one by one, feet followed by hands. In each, a contact button fires the compression spring, placing a good deal of tension upon each limb. Circulation may be inhibited, but only for a short while.

When the last spring is fired, the stepper advances, the shaft brake release latch is triggered, the carousel around the main shaft, tensioned by the pre-wound shaft coil spring, then starts rotating, rapidly gaining a rotational velocity sufficient to fire any hangers on or in any significant way improperly tethered prisoner into a wall studded with 12" sharpened spikes anchored upon 3" centers (the chamber is cleaned out nightly, or as found to be necessary). This is the main sortation phase.

After an interval sufficient to effect any necessary sortations, the stepper thereupon advances, firing the carousel brake engage. The brake arrests the carousel rotation moderately quickly. After a delay sufficient to ensure the shaft is stopped and vertigo has subsided, the stepper advances, the irons compression springs are unlatched, and the prisoner may disengage themselves from the irons, turn around, now prepared for the final exit sequence.

Being careful to avoid the aforementioned spikes, the prisoner then pushes the clearly marked external door release button, and is on his way. After the final delay, as previously described at the outset, the external door closes, the stepper resets to its initial state, the chamber now ready to service the next emancipation cycle.

If near London, please stop by our dimly lit, poorly ventilated & extremely drafty workshop. We forge, monger & mend a full line of dungeon hardware.

theRiley
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    Huh...pretty cool! – Radvylf Programs Oct 03 '18 at 14:51
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    Welcome to Worldbuilding! I think this answer could benefit from an explanation of why the mechanism described meets the question criteria. – Jared K Oct 03 '18 at 14:53
  • @Jared K - you are probably right on the timers,this is more late renaissance tech. the springs i think are remotely realistic. the main thing i wanted to tackle was a repeatable system not requiring guard intervention (beyond daily cleaning) guaranteeing singletons. let's substitute glass timers contour-weighted to invert on empty, landing on actuators :) – theRiley Oct 03 '18 at 15:09
  • just thought of another potential problem. two inmates ruggedly sown into a single garment would cheat the system. So I will stipulate that any sort of even potential sewing contraband found during periodic cell searches will be met with slow, painful death -or- all inmates are kept naked, kept well clear of any cloths or fabrics or hides or fibers of any sort whatsoever. – theRiley Oct 03 '18 at 17:14
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If he doesn't want to devote any of his soldiers to the task, another prisoner could do it instead. This prisoner would be tasked with monitoring the gate, and ensure that only one person is released. This is how he/she would earn his coins.

To prevent people from forcing their way through once the door is opened, the easiest solution would be to have an outer and inner door. The outer door cannot be opened until the inner door is closed, and the inner door won't be closed if more than one person has attempted to enter.

To discourage the gatekeeper prisoner from cheating and deliberately allowing more than one person through, the gatekeeper prisoners could be kept isolated from the other prisoners at all times. They could also be told that soldiers guard the outer door.

user1751825
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    If I as a prisoner control the gate, I don't need no stinkin' coins - I get a free pass. And I'm pretty likely to let everyone else out too since there are no guards there to prevent it. Even if the prisoner-gaurds are in a separate compound, there must be a way to get to the gate if my own coins are ever to do any good. – brichins Oct 02 '18 at 16:49
  • @brichins it might work if the prisoners believe there are guards outside the gate. Even if there really aren't, they'd have no way to verify this. – user1751825 Oct 02 '18 at 21:21
  • @brichins or, you have 2 completely isolated groups of prisoners. Each group operates the gate for the other, and each are told that they cannot get their coins or eventual release unless they follow the rules. – user1751825 Oct 02 '18 at 21:25
  • Two groups is basically the opposite of the prisoner's dilemma - the worst possible personal outcome for group A to not help group B. If you have guards, they're the actual control system; if you have no guards, prisoners have no reason to not just open each other's gate and let everyone walk out. – brichins Oct 02 '18 at 22:20
  • @brichins This risk could be mitigated to a certain extent by preventing any communication between the groups. The groups would have no way to coordinate the release, and no reason to do the other group any favours that might not be returned. Then there is the threat of being discovered by the guards they believe are patrolling outside the gates. Finally, if the gates require a human to operate, then there is always the risk that you may be releasing the only person who can release you. – user1751825 Oct 03 '18 at 02:15
  • @brichins Also if it's an inner and outer door setup, then the process of actually releasing a prisoner may be a fairly slow, cumbersome process. You couldn't just open the doors and let them all make a run for it. The most you could do is release a few at a time, but then you might be risking your own freedom. – user1751825 Oct 03 '18 at 02:30
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    creating guards avoids the problem. – theRiley Oct 03 '18 at 14:56
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Let's get a couple of things out of the way. . 1. any door that works for a large person, will work for 2 smaller people if they squeeze into a fat man's clothing.
2. Without human, all mechanical defenses can be defeated. Let's ignore that.

The only way to make this work is to give all your prisoners ankle bracelets. If a bracelet is ever removed, they explode, or the army charges in and executes the violator.

Next you have a terminal. If you earn enough coins, you can go in, put in the coins and type in your prisoner number. This deactivates the bracelet.

Next you have a 100 foot hallway with 2 airlock doors. Both doors can never be open together. If there is anyone in the hallway with an active bracelet, the second door cannot open.

So when prisoners who are freed go in, they can open the first door. Close it behind then, then walk over and open the second door. If anyone with an active bracelet gets into the hallway, it can't be opened.

Your bigger problem may be people going the other way as they don't have bracelets and could travel back and forth freely. A simple 1 way revolving door with crossing bars as a filter like in the subways solves this.

Andrey
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  • Since this is supposed to be medieval tech, you would want clockwork dials or something, probably not a keypad. A bracelet that explodes if removed sounds very tricky to make reliable. But I can't think of any tech even close to medieval that would detect whether a bracelet is within a region or not. – aschepler Oct 08 '18 at 14:30
  • There would be no way to make anything like an exploding ankle bracelet using mediaeval technology. Even ignoring the complication of detecting proximity. At best they may have had rudimentary knowledge of black powder, but that would be far too bulky. Also the only way they could have ignited powder would be to use a flame. A prisoner could defeat this simply by soaking the powder. – user1751825 Oct 08 '18 at 22:32
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Once a day (or once a week if manpower is that stretched)

Reinforced "airlock" cages. Each cage is large enough to moderately comfortably hold one person (has a chair included) and can be accessed by appropriate tokenage. Once a day at a designated time a guard comes, overlooks the cages, and assuming there is only one person per cage locks the entrance sides, then unlocks the exit sides and all the liberated prisoners are free to go. This requires only a small amount of effort on the guard's part (maybe 5 minutes?) while still ensure only appropriately paid prisoners are let out.

aslum
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We have several answers that touch on air supply but I don't see that they are adequate--either the device could be shared or how do you calibrate it?

Thus:

You put your tokens in the slot and then put your head into a cavity in the ceiling that air is slowly being pumped into--enough to sustain a person. There is a spill valve on the top of the cavity that will dump the air, this value is normally open. To avoid losing the air it must be pressed (not a lot of pressure is needed) closed--and given the nature of the device the only available means of doing this is with the top of one's head.

If two people try to cheat when they swap all the air in the cavity is spilled out. While there is enough air being supplied even for one large person (which might suffice for two small people) there isn't enough for two given that you keep losing it every time they change places.

Loren Pechtel
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In medieval tech, perhaps there are vicious poisonous/deadly thingies, with known antivenom/prophylaxis sources. These don't have to be snakes, perhaps there are lethal mosquito-bourne parasites (a lethal version of malaria/ebola?) and some local plant or product that's been found to confer immunity or a cure, when eaten.

Your token gets you enough antivenom/prophylactic substance/antidote for one person. The exit room is designed to lethally affect anyone entering it. Problem solved.

(On an overview, you're looking for things that are definitely able to be rationed one per person. Air is an obvious solution, breathing gear too. Really, anything that you need a certain amount of to live, and a situation where two people trying to share the resource will die, will answer the question. But I prefer toxins to air/water. So pretty, those blue agonised deathly faces, when you dump them back in among the prisoners with the tag attached "another person who thought they could beat the exit token system" ;-) )

Stilez
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  • I was about to write this exact answer. Here are two obvious issues people might think of and why they are not really issues: 1. poison prevention - prisoners will try to shield themselves from the poison. Fortunately there are many kinds of poison, many of which require more resources and knowledge than medieval prisoners have to shield from. 2. Diluting/sharing the antidote - I am no toxicologist but I think antidotes quantities work based off weight. Trying this hurts the chances of the other plus any failed attempts are immediate death, again they are ill informed, medieval prisoners – Jesse Oct 05 '18 at 05:56
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This is a problem of supply and accounting rather than technology and doors.

Each prisoner's stones needs to be collected and brought to wherever you need them. During this collection his stones are counted. This can be done by giving each prisoner their own section with a name or code associated with it, and dogtags that have that name or code on them. Or if you want to risk people thinking it's a concentration camp regardless of how well treated the prisoners are, give them a tatoo so the dogtags can't be stolen. You could combine it: Dogtags have a name and a symbol on it, as does the section. The prisoner gets a symbol tatoo and a dogtag so people can't steal dogtags and collect more coins.

After hauling the stones to his section, the prisoner is tasked with stacking them in a particular way that allows the collectors who come with carts to do an easy count. Then the prisoners themselves are tasked with helping load the cart with the stones, once finished and no foul play is found they will get their coins based on the amount of stones found in a pile. This means that it might take a few weeks for people to get their coins as transportation might not come every day for their particular pile, but they will get them.

If you have limited space, for example because the stones are hauled to a warehouse, the prisoners are put into groups with dogtags. The group as a whole brings stones and puts it in their section all stacked and ready for transport. Once on the carriages the group gets paid with everyone an equal share. This promotes self-regulation of the group, when one slacks off the group will be punished with slower coin gains. Should the slacking off the too much the group might punish that person by stealing a portion (or all) of their coins after collection. Groups also help self-regulate stone stealing. For example an enterprising person who decides that hauling stones so far isn't worth it if he can steal from a pile nearby. Groups can police their own section, and with proper labor division there's always someone stacking stones and guarding it from intruders while the rest is hauling the stones.

Once enough coins are collected, the prisoner can pay the guys who give the tokens in the first place (easy recycling).

The beauty is that this system uses the simple things you'll be needing anyways: people who collect and distribute the work (not just stones as mentioned) and who can assertain how much work was done upon collection and give out the reward. If these people are corrupt then the collection and transportation of the work would already fail and theres no worrying about prisoners getting loose too quickly, and those people would be more inclined to keep their prisoner labor high than let these people out.

Demigan
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Door is not enough

If there are no guards, you can't just have some walls and a door to keep prisoners inside. I mean, what would you do? Carry stones, or start using these stones to build a ramp over the wall?

Island on river

You need to have your prison on an island in the middle of a fast flowing river. Add some alligators and cliffs if you wish. It is then impossible to escape without help from the outside.

Paying passage

The king is not willing to pay for guards checking that nobody gets out. But, with such a setting, you could have some guys living on the other side of the river, and every day they go to the island by boat. If someone has enough coins, they can buy a passage.

The boatmen

The people that are going back and forth with a boat are also punished by the king, but they were part of the nobility. If they bring back enough coins to the king in the capital, they gain the right to get back to their previous status and privileges.

This way you fulfill everything you wanted:

  • Only the persons that can pay are going out (the men that own the boat are not going to give free rides to anybody)
  • No guards. The people that own the boats are also prisoners, but with a better status. In order to gain more coins, they must avoid prisoners getting out without paying.
  • No infrastructure (cheap)
  • No real money involved. The coins are just pieces of wood/lead/cheap stuff. When they are brought back to the king, he can just send them back to the starting point.

Bonus ideas

  • If you were not a prisoner, but that you bring back many tokens to the king, you can have your status elevated in the nobility system, (from viscount to earl, or from nothing to knight for instance). So you encourage ambitious people to work there as free guards.
  • The people on the good side of the river could also sell food/beer/clothes/... to the people inside for jail-coins. Like this, even if you don't want to go out, you must still carry stones in order to eat.
Legisey
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  • What stops a bunch of prisoners on the island from ambushing the boat man, and stealing his boat? – X-27 is done with the network Oct 04 '18 at 18:53
  • The swords of the boatmen. And their friends the archers from the other side of the river. And the other prisoners not wanting to loose the chance to have a boat taking them one day to freedom. – Legisey Oct 04 '18 at 19:40
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The door only opens with the appropriate key. An armed patrol comes by once a week, counts the tokens, and lets the appropriate amount of people out.

If that's still expensive, reduce the frequency. Once a year would still not discourage them too much.

The door is narrow enough to mitigate massed attacks from desperate slaves.

Emilio M Bumachar
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Another option - a chamber with just enough air for one person that the prisoner stays in for several hours.

The prisoner puts the coin in the slot, door opens. He steps into a tiny phone booth sized room. Door closes (and seals). He stays there for six hours - or for however long the air there would last for one person, but not two. After which, a door on the other side slides open. You'd include suitable mechanical checks to ensure that only one door could open at a time.

If two people try to fit into the room, they'd both suffocate. (And truth be told, some prisoners who are freed may die if their oxygen consumption is too high because they are too excited. But, them's the breaks.)

The evil dictator would have to do a lot of experiments to get the proper size of the room that balances out maximizing single prisoner survival and minimizing double prisoner survival. Many brave 'volunteer' test subjects would die in the process. But that's a small price to pay for the glory of the empire.

Additional thought: To handle, in an automated fashion, those prisoners who die during the process and don't exit the room after the wait time is up, you could have the 2nd door actually be the floor. After 6 hours the floor drops out, dispensing the freed prisoner into a river below*. This way the other prisoners would never see a dead body of someone who was freed but suffocated.

(* - you'd want a river so that bodies don't pile up below the door, plus it offers a soft landing)

GrandmasterB
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  • I'm not sure an airtight room is within the technology level the querent is shooting for. – T.J.L. Oct 03 '18 at 14:59
  • I don't think this is any less feasible than some of the other potential solutions. The whole thing has to be coin operated, which imo would be the far more complex feat to do in a purely mechanical way. – GrandmasterB Oct 03 '18 at 17:44
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Kind of building off a few answers: You insert coins that drop them onto a scale. The counterweight keeps the trap door closed, if there is not sufficient counter balance in coins the trap door is opened and you can't walk to the exit. When the exit door is opened, the scale is tipped and dumps all the coins off it and resets the trap door.

Jordan.J.D
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Water clock / watersupply

A variation on already suggestion solutions combined with my own ideas

Have a mechanical deterrent to show it's only meant for 1 person. It's clear now it's only meant for 1 person.

That gate or the closed path after it is on high ground. Then you enter a room with a see-saw mechanic. This starts a 2-3 day water clock that locks you in the room. During these days the water supply is limit meaning that a normal 'healthy' person could survive but not 2. (maybe 2 kids can). It's similar to limit air supply but less air tight issue/ reset issue/ hyperventilation issues.

As an extra you could add a 6 hour waterclock that starts with a loud bang and opens the floor. But giving locked up people the time to grab onto some foothold. If they aren't strong enough or have passed they fall in the river below.

Double Bonus the fallen bodies get carried by the river to the lowerground where the camp is. People will catch on that if 2 enter none survives and if 1 enters they probably most of the time will.

dwana
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  • Welcome to Worldbuilding.SE! We're glad you could join us! When you have a moment, please click here to learn more about our culture and take our [tour]. Thanks! – JBH Oct 04 '18 at 14:12
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Prisoners collect coins to buy an antidote for a poison.

The door that leaves the prison is left unlocked. Anyone can leave but they must walk through a long tunnel filled with a poison gas. Only people who drink the antidote survive the tunnel that leads to their freedom.

A flask of the antidote only contains enough for a single person.

Reactgular
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Issue prisoners a one-time identity token when they enter. All identity tokens are identical.

Exiting requires you to submit an identity token AND the required coins in two separate slots.

ghosts_in_the_code
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I think that in order to find the answer we should be thinking out of the box. It shouldn't be about a smart sophisticated system. It should be about synchronization and the psychology of the enclosed enemies dying.

Think that you have 2 imprisoned that each have half of the amount required to open the door but summing the money they have the exact amount needed to open the door:

1. Psychology

If the risk of both dying is high then the imprisoned will not even think of trying to escape. Since they do not do any work in the camp they will prefer to wait until each one has its own amount of money necessary to leave normally. Anyway, they are not being tortured or forced to work inside the camp. As such they will probably prefer to delay the escape to do it safely.

2. Synchronization

Think of it. Russian ballet dancers spend hundreds of hours in order to synchronize their movements. A racing team (F1 for example) spends hundreds of hours in the pits in order to synchronize their moves and fill the fuel tanks and change the tires of the formula car in seconds.

Now, think of a system that permits one 'even' over-sized person to pass. Someone (in previous post) posted the following image which I like for such a system. enter image description here The system should be having the following ability. If the door after opening does not close within 2-2.5 seconds, then a guillotine falls and kills anything inside the enclosure. The time of 2-2.5 seconds is enough for someone to open the door, enter the tiny enclosure, close the door and leave if s/he is fast. But, what about 2 people trying to escape together? The added overhead of the synchronization needed to complete successfully the mission, should theoretically kill them.

Even if there is a chance that the door does not kill them, then just the psychology of having increased chance of dying should stop them even from trying.

Basically, the solution to the problem seems to be about the possibility (mathematically) of surviving.

0

I know you said medieval, but this might require a bit of "Rube Goldberg" technology...

Just beyond the door is a pit full of poisonous spikes. There is no way across the pit except for a stepping stone in the middle just wide enough for one foot. Stepping on the stone causes the door to slam behind you and there is no space between the door and the pit. This way, anyone waiting for you to get off the stepping stone so they can use it will be knocked into the pit.

As an added bonus, any weight on the far side of the pit also causes the door to slam closed, in case some particularly athletic slave tries to jump over the stepping stone to avoid triggering the door close mechanism.

komodosp
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If you want to be evil, then be evil in the real way. The door is just a concept. It does not really exist as a door. Rather, it is in the shape of a gun, and one token pays for exactly one bullet.

Get out of life card (Source: http://www.blastwave-comic.com/index.php?p=comic&nro=14)

This being a medieval setting, you can replace the shotgun with a crossbow if you are not into gunpowder.

The Square-Cube Law
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Have two separate groups of prisoners, Red and Blue.

If you're in the red group you need 10 of blue group's currency to escape, if you're in the blue group you need 10 red group's currency to escape.

The door to freedom is regulated by the prisoners, if you're in blue group, a person in the red group lets you out for 10 red group's currency thus paying the red group person who let them out, if you're in the red group, a person in blue lets you out for 10 blue group's currency.

The two groups are isolated and to buy one of the other group's currency it costs 10,000 of your groups currency.

Merchants come and go every so often, they trade in the correct currency for the group their trading with, they then trade their red/blue earnings with the dictator so currency self circulates.

Thus the one person door is the other groups desire for freedom.

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    I miss how this is answering the question. – L.Dutch Oct 03 '18 at 07:22
  • the original author said the only problem is that he needs a door to release exactly one. He can't afford to send a soldier to guard the door my idea is the prisoners control the other groups freedom by tokens that is the other group's currency – Thomas Eisner Oct 03 '18 at 08:36
  • @ThomasEisner Please should edit your answer to reflect this, as it's not really clear in the answer itself how it solves the problem of only letting one person leave at a time. Also, why would people from Red group stop people from the Blue group from cheating the system (e.g. 2-3 people getting out instead of 1), when they could help them and in return be helped by them? – Hankrecords Oct 03 '18 at 09:33
  • i see your point and have realised a flaw in my logic. thank you for your feed back. as blue could pay some blue groups currency that is worth more to red for for freedom – Thomas Eisner Oct 03 '18 at 10:41