Your territory has been invaded by zombies. Unfortunately, there is nobody else to fight, so you have to fight them off all by yourself. Prevent them from passing your tower this night. Each hour is compressed to one minute, after which you may buy additional weapons.
Use your mouse to aim and fire. Press the space bar to reload your weapon. Press G to throw a grenade and change weapons with the scroll wheel or the arrow keys up and down. You may also change keys in the menu.