Here's what is written in FizMig: Fortification defenses of the city and their assault. The formula for calculating the base damage dealt by the central tower of a specific city is as follows: Damage = D + d * N where D is the base damage of the central tower = 10...15 units d is the additional damage that the ranged tower gains when building another "structure" N is the number of "structures" that have already been built in the city. In an "empty" city (Rural Settlement and Stronghold), the central tower deals 10 to 15 units of damage. Each built "structure" adds 2...3 units to the possible damage dealt by the tower, bringing it (in a fully built city) to 46...69 units (for example, in the Dungeon). It should be noted that the actual damage often exceeds the upper limit, but never goes below the lower limit. Different types of cities have a different number of "structures" and, therefore, different tower capabilities when they are all ("structures") built.