In order to quickly break down the defenses of an enemy town or fort, your creature stacks have the option to attack the door and the walls. Sometimes my creatures do 1 structural point of damage, while at other times they do 2. What determines how much structural damage a creature stack does?
The damage is determined by the physical size of the unit.
A unit that needs one tile does 1 point of damage. A 2x2 unit does 2 points damage.