Ballista and towers.

How is the damage dealt by a ballista and a tower calculated? I remember that when defending my Castle, my towers dealt 0.5-1.5K damage with an expert ballistics skill. How is the damage calculated exactly?

The amount of damage depends only on the number of "structures" (buildings) currently in the city. The defense level of the attacking unit and its position on the battlefield do not affect the amount of damage.