So I'm in the 1700s and thinking of building a castle surrounded by a 1000 metre moat.
I am going to section off the moat into three parts so that each section contains piranha fish, crocodiles and sharks.
That should keep me safe, shouldn't it? Try and swim across and be prepared to meet your maker.
No capacity back then to have someone drop in on me via air so it's all gravy.
How would you get to me given my defences?
Having an impregnable castle does not equal having an infinite supply of food. Unless you learn how to survive on light and water, anyone bent on conquering you can just keep supplies from going in.
This will be specially though on you since keeping sharks, piranhas and crocodiles will require a lot of supplies (they need to eat too). And that's handwaving the fact that you would be able to have those animals in moats - even nowadays it is hard to condition zoo and water park water conditions for crocodiles and sharks. Piranhas can be kept in a modern day fish tank, but that requires technology not available before the mid to late 1900's.
Also notice that the purpose of a moat is to make it harder to get to the walls, not to make it impossible because of critters. Even if you managed to maintain an artificial environment for those animals, an attacker could just throw salt or **** in the water and your aquatic menagerie would be disrupted into toxic death in a matter of hours. To add insult to injury and injury to insult, I would also burn manure upwind of you just to spite your castle staff.
The old medieval Siege and Wait ended by this time. If you really want an idea of a defense of that time, you should learn about the Siege of Vienna in 1683. In that siege, a vastly outnumbered Austrian force held out against a massive Ottoman army. Finally, Poland's winged hussars broke the siege.
Lessons from the Battle: It was the troops that were the best defense. They could only send specialists, and used complex tactics to buy days in a hopeless siege. Things like enemy groups of sappers running into each other as they tunneled, and complex trenches to limit artillery deaths.
A Simple Wall or Moat
Would likely just get steam rolled by any professional army of the time. Think of something like the piranhas and crocodiles so they need food, which wastes yours, but if they are in a hurry, they could just position the moot, fill it with rocks and dirt, or just order your men to build pontoon bridges. (if the animals could even survive). Then say charge after a constant artillery shelling. Etc.
"That city is well fortified which has a wall of men instead of brick." - Lycurgus of Sparta
-I fish your piranha and sharks, hunt your crocs, that's free food...
-build a few quick mill to drain all your water to flood your forests and agricultural land
-Trebuchet people infected with the black death to your castle... you gotta come out and clean up the mountain of dead that will be piling at your door some day, and when you do, you'll be doomed.
-Enjoy my fish and meat salad while I watch you despair
Are you sure you want to build a CASTLE in the 1700s?
A castle is typically a medieval structure build to withstand the medieval combat, not the black powder powered artillery pieces. Moreover the walls simply do not have the space to place heavy cannons on them so the best defenses you've got are some light artillery and maybe mortars. This means that you are outgunned and most likely outranged, even with you altitude advantage.
What I would do is simply fill the moat to the point of getting my guns in range and just bombard your positions until it turns into ruble. Which shouldn't take a lot of time since, again, a castle is not build to withstand heavy bombardment from artillery.
