Well, the joke's on me ! I like the wall idea, but I suppose a physics engine would be a little too much to ask of hobbyist freeware game. It would be nice though... In case you are using AoK and AoE style walls, you could resrtict which units would be able to attacl walls. For example, melee and cavalry units would not have any effect on the walls, archers could do some minimal damage, and siege weapons do the most damage.