Infected skull pods are trying to infiltrate your camp and contaminate your base. Blow them away with just a shot from one of your powerful turrets. The skull pods will try to confuse you. Your turrets will switch the direction they shoot with each level. Retrain your mind to see before reacting and you might be able to annihilate the skull pods before your camp falls to them.
Protect your base by shooting down the deadly skull pods before they can reach you. The first level is deceptively easy. Press the up arrow to shoot up, the down arrow to shoot down, etc. However as each level progresses the arrows will change. You may need to press the left arrow to shoot an enemy coming from below.
If a pod makes it to the middle, you will lose a life. Guard them fiercely, you only start with three.