Your job in each level, and there are a lot of them, is to destroy and kill
all zombies using different weapons. For each killed zombie, you will
earn some money. After each level you can go to the shop and buy some new
and better waepon to get more power and to kill the zombies as fast as you
can. You can also repair your barricade using some stuff from the shop.
But for all of this you need money, which you will earn if you kill enough zombies.