In the final stage of the game, you must use the ship that Maria gave you to blow up the fortress. This stage is unlike any previous stage. The view is presented from behind your ship, with the fortress off in the distance.
The target of your attack is the circle in the center of the fortress. All of the shots fired at you come from this circle. In order to avoid getting hit, you should move constantly. A cross-hair is visible on the screen, which shows you where your shots will ultimately land. When your ship is below this target, the cross-hair will appear above the ship. If you move such that your ship is above the target, the cross-hair will immediately appear below the ship.
In order to safely destroy the fortress, you should move your ship in a roughly circular pattern, giving your cross-hair a chance to pass in front of the target circle, and firing whenever that occurs. After you fire, continue your circle around and allow the cross-hair to switch sides, taking the opportunity to strike the fortress again on the other half of your circular trip. Keep moving so that the bullets don't have a chance to lock on to your position.
After enough successful shots, the fortress will explode, and the game will end. You will get to watch the ending (in the Japanese version, there are three possible endings determined by your response to a question), and then you will receive two secret passwords.