Grab your gun and take down incoming zombies one by one. Headshots are more effective and you save ammo earning you more points too. Shoot health aid kits, ammo bags and explosives dropped on the ground to help you last the time necessary to hold your defense in order to complete each level and use the money on the shop to buy better weapons making more damage.
Use the mouse to aim and shoot. Press the Spacebar or R key to reload.