Fortress of Zorlac is a shoot 'em up with some similarities to Yars' Revenge. The player's objective is to rid the galaxy of a dreaded alien called Zorlac. Zorlac is positioned on the right side of the screen and is protected by a fortress consisting of three rotating walls as well as four indestructible stationary guns. The player starts out at the left edge of the screen but can move around in all directions. To be able to hit the alien the player first has to blast a hole through the three walls. It is made harder by the constant movements of both the rotating walls and the alien. At the same time the player has to avoid being hit by the defense guns which automatically lock in on the player ship. As the game goes on the fortress slowly moves toward the player who gets less space to dodge the enemy fire.

Once the player has been able to hit Zorloc, he gets enraged and the game moves on to the next level that's played at a higher speed. The player scores points by destroying the walls of the fortress and for hitting the alien. Points are awarded based on the skill level chosen before a game and the player's distance from the edge of the screen. Hitting the alien is worth fifty times more than destroying a block in the fortress. The player has access to five ships and once they have all been destroyed the game ends.