Instructions
Buy better weapons. You start with a pistol that is practically useless. Kill zombies with it to get money, then flee to the weapons shop along the wall. You can purchase better weapons there. Money from defeated zombies is automatically added to your total.
Enter one of the bunkers scattered along the game area. Bunkers are fortified against the zombies, making it harder for them to get to you.
Shoot the zombies from inside the bunker. The only time to leave is if you want to go into a zombie cave to rack up quick money to buy better weapons, or to buy better weapons. Staying in the bunker is the best defense you can have.