Survive as long as possible by building the most efficient line of defense. Eliminate the bad guys from attacking your home base. There are also several different game modes available.
Shoot the incoming enemies to protect your base. Collect weapons and turrets that automatically fire on the incoming missiles. Push the turrets around the battlefield to take advantage of their range and abilities.
Defend the tower as aliens are in full force of attack. Upgradable. Destroy as many aliens as possible and earn more credits to purchase/upgrade equipments.
Strategically place your army against waves of enemies. Shop for new weapons of destruction at your local War-Mart and upgrade them to deal maximum damage!