i'm not an expert in raiding
but in order to prevent walls stacking isn't build a build like "fort" or "castle" much effective??
the game is placed in another historical period but i know in middle ages the cheaper way to protect the city was an armored place over wich left "outside" the buildings
that sort of fortress can be modular and with ladders to reach roofs in order to shot people attacking, as well it may have the gates ables to be locked
maybe i'm offtopic? i hope not