I've often heard that one way castles were taken was by poisoning their water supplies. What would have been used? Would it be specially made poisons or just stuff like feces?
Frequently it was corpses. There’s no need to mix up huge batches of special poison when you can just dump the bodies of animals or men killed during skirmishes into the water. During the siege of Acre, for example, Saladin’s forces threw the bodies of the dead into the rivers because the Frankish forces were downstream.