When zombies are in really dense groups like when they form up at the base of a wall while you laugh from the 2nd floor, the ones in the center of the group should slowly start to rise. As though they are standing on fallen zeds. Once you get enough bunched up, the pile should get so high that a few can climb onto the 2nd floor or hop a fence.
This would make certain areas that are enclosed by natural fences less powerful. And would let us increase the durability of walls since there is now the chance for zombies to surmount them.