I built on pretty level ground on foundations with ramps going up to the new ground level, a giant square. In the center is a tower that uses walls for the first level, making an octagon shape, I fight from atop this mid-level or the roof where the heavy firepower is. When I am on the top of the tower or in the middle, all zombies line up around the perimeter and beat on the stairs that surround the base. If I go to the middle level, same, if I go to the floor level, or what is the floor if you go up the ramps, all the zombies then path to the base. As soon as I go back to the middle or upper floor, the zombies go back to the ramps and start beating on them at ground level again.