In Alien Guard you're in charge of a massive cannon gun that will shoot special bullets that kill the aliens invading your base. Use the mouse to aim your gun and click to shoot. The up arrow will allow you to jump and the down arrow will allow you to duck. You cannot move from your post on the top of the blocks. You'll have a health bar and every time an alien jumps on you, you will lose a health bar. The idea is to stay up there and keep killing as long as you can before you run out of health. How many aliens can you pop a cap in when you play Alien Guard?