The game takes place in modern times where a great depression has swept over the world. Amidst the depression and its riots of jobless workers, a mad scientist starts his plan of world domination. Taking command in the titular fortress orbiting Earth, the evil Doctor Gegeben Funkerun uses his own robots, flying fortresses and mercenaries to wreak havoc across the globe until all his demands are met. Assisting the attacked and weakened governments, workers across the nations help develop a series of jet fighters called the Aizerun Geist to use against the forces of Dr. Funkerun and save the world.
Before each stage begins, players are given a choice of weapons to purchase at a hangar for the Aizerun Geist. There are four weapons within three tiers to choose from including Main Attack, Sub Weapon and Bomb. Of the Main Attack the player can choose Twin Shot, Laser, Heavy Laser and the Charge Shot. Of the Sub Weapons, players can choose additional firepower, Homing Missiles, Shielded Options or an energy barrier. Players can stock up on multiple bombs including Small bombs, rockets, larger bombs or napalm.
The Aizerun Geist provides tactical advantages during combat with the use of its manually directional Sub Weapon. Players are able to direct additional firepower in five directions including vertical and horizontal by pressing the bomb button while firing. The game runs for six stages through various settings such as a jungle, stormy skies, Tokyo and Fortress Strahl itself.
