Space Fortress puts you at the guns of a solitary space fortress being besieged by a swarm of alien ships. The game may be played by up to four players taking alternating turns.

Your fortress is located in the center of the screen. Alien ships will appear at the edges of the screen. You must blast the aliens before they hit your fortress with their fireballs. Kamikaze ships will sometimes appear and attempt to destroy your fortress with a ramming attack.

At the beginning of the game you choose how many fortresses you would like to play (up to ten.) The players may also choose from ten levels of difficulty. ~ Joseph Scoleri III, All Game Guide