Ballista and towers.

A couple of years ago, when I was looking into this, I found something online stating that the damage of Towers depends, firstly, as already mentioned

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

Also, the damage of Towers increases each week while the player owns the Castle. As soon as the Castle is recaptured, the damage is reduced by half and starts to increase again. Thus, in long games, the damage in the initial Castles of players can easily reach 2000 or even more.
However, this is not in WoG, but either in SoD or in AB.