Aliens are attacking your planet and its cities. You must defend the cities by destroying the alien ships. Using the mouse to aim, launch missiles at the ships by pressing the A key for the left launcher, the S key for the center launcher, or the D key to fire a missile from the right launcher. The launchers and the cities are arranged at the bottom of the screen on the planet’s surface, and are destroyed when the alien ships collide with them. The alien ships are destroyed when struck by your missiles. At the end of each round, bonus points are awarded for unused missiles. At higher levels you will receive more missiles, but the alien ships will advance more quickly and in greater numbers. The game ends when all of your cities or all of your launchers have been destroyed.