Ah, free to play games: they really do seem to be taking the world by storm. Sort of like zombies. So it makes sense for a game in which kill horde upon horde of the walking dead to be free to play... or something. Anyway, Zombie HQ is a fairly popular game on iOS, and the developers just dropped an Android version on the Play Store.
Zombie HQ is a game in which you shoot zombies. It's an overhead run-and-gun shooter, if you want to get picky with the details. It looks reasonably well made, and its 4.5 star rating on iOS would seem to support that inference.