Wall that cause damage. Maybe only 1 damage per 5 seconds, but that might change the battle. Maybe the walls could be loaded with gold, as that when at top levels is only used to train spells. When fully loaded, the wall will damage attacking troops, sort of like the effect of poison. When wall is destroyed though, it causes no more damage.
Possible ideas to add on are:
When wall is destroyed, remaining gold inside of wall (gold not used to cause damage) is added as a loot bonus. However only about 15% of what is left in the wall at the time of destruction.
Super Walls
