Thats because OhGreat was a drawing hentai at first, so he always uses a lot of fan service.
Wouldnt a huge stone wall be enough to ward off the zombies? Since they dont seem to attack nonliving things. So you would need a huge group of people to build a wall near a river or something else where you dont need a too big wall to get a huge zombiefree area.
But I guess that building such a wall would use quite some time, so hmm, how to survive against zombie masses until it is done?