one way to implement this "horde attacks your base" thingie would be to use a scent map. Having a permanent base implies going to and from lots of times, thus leaving a scent trail behind you. Zombies could sniff your scent when it grows strong enough, and follow it to your base. But since they're active only in a small area around the player, maybe just have a horde spawn along those routes every now and then would simulate good enough the fact that they're supposed to follow that scent from wherever they are. (all scent map idea is actually borrowed from CDDA, but i'm sure no one would mind )