Zone Mu "Great Fortress I" is Stage 5B of the game. It is a flying fortress in space. This zone splits in Areas W and X.
Boss 5B is Heavy Arms Shell, a Flying sea turtle in the stars.
See Heavy Arms Shell article for more information.
The ending of this zone shows both Silver Hawks escaping from the exploding Heavy Arms Shell, but Sameluck pushes Lutia away, leaving him to die. Then it is revealed that Lutia's Silver Hawk crashed on Darius' surface, Lutia lies on the ground crying because of Sameluck's death, however a mysterious man gives her a hand and she accepts, which implies that Sameluck survived the explosion.