The invasion of zombies is probably the most terrifying thing to imagine. What would you do if you were surrounded by hundreds of zombies? Kill them and upgrade your weapons.
Protect the building you are in and eliminate all incoming zombies. this is your shelter and there is no other place you could go. Get ready to face enormous waves of zombies and kill them before they destroy the fence. Reload and shoot accurately and when the level is finished, spend some money on various improvements.