I'd point at the MoM sieges
Destructible walls. Put in a destructible gate that can be attacked by anything. Make it so that the walls cannot be attacked by anything but a spells or a unit with the "siege" characteristic.
This wall could go around a tactical map area, or just be in the middle, a-la Dominions. This wouldn't have to be a terribly sophisticated siege mechanic, just a way to make units in towns "safe" against normal fodder and monster. I'm picturing a few ranged units in a walled town easily fighting off any random gaggle of melee monsters/units, while "big" monsters (with the "siege" trait) and prepared armies pretty much roll over the town.