Very simple and repetetive game.
Normal mode was very easy, completed it first attempt no problem.
It seems to me that anything above normal mode is simply impossible to complete - your guns can't generate enough ammo to kill them as fast as they spawn. Ok maybe that is the point, not all games have to be 'completable' but more annoyingly than that - as soon as I had more than 15-20 zombies on the screen it was causing some series lag.