Ballista and towers.

The number of buildings certainly affects the power of the Tower's attack, but it is far from the only factor.
I've seen such wonders that Ley talks about; Towers can simply obliterate a huge number of creatures. I managed to defeat 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 you've repelled.

So: I don't know the exact answer, but regarding the buildings in the city, the answer is not exhaustive.