I've seen such amazing things that Ley talks about; Towers can simply obliterate a huge number of creatures. I managed to take down more than 10 Black Dragons with a single shot from the central Tower.
What does it depend on? I don't understand. It feels like there's some kind of cumulative bonus based on the time you've owned the Castle and the number of attacks it has defended against.
So: I don't know the exact answer, but regarding the buildings in the city, the answer isn't exhaustive.
