In most games zombies must be shot, trapping them or somehow overcome. In the current do not. It's probably after the battle, mankind has won and now only addresses how to deal with undead. Someone invented a lot of different environments, where the zombies will be disposed of and entrusted it to you. In each level you have a few ways you can dispose of the survivors and you have to use every way to advance to the next level. Maybe a little harsh, but better than if it was vice versa.