The X-Naut Fortress is Sir Grodus' main base of operations which is located on the Moon and the home to the X-Nauts in Paper Mario: The Thousand-Year Door. Princess Peach is also taken to here. The last Crystal Star, the Crystal Star, is held there and Mario and his friends have to use the cannon in Fahr Outpost to reach this place and fight Lord Crump (while controlling Magnus von Grapple 2.0) to get the Star. Grodus travels between the Mushroom Kingdom and his fortress using a Teleportation Machine, which has one end located in his laboratories and the other end located in Rogueport Sewers. The base is also the home of TEC-XX, which is the main computer of the whole fortress. Also, after Mario gets the sixth Crystal Star, Grodus takes Princess Peach and leaves the Fortress (possibly giving it to Lord Crump) so he can go to the Palace of Shadow. After Mario successfully destroys Magnus von Grapple 2.0, he gets the seventh Crystal Star. After that, the fortress explodes and Mario is unable to go back until the final boss is defeated.
The fortress is essentially a five-story building built deep under the Moon. It is all connected by a series of hallways and two elevators. The deeper Mario or Peach go, the dimmer the lighting is. The in-game map description reads, "The fortress of the dubious X-Nauts. Their leader, Grodus, lives here."
When the player Spring Jumps into the air duct from the Yux Lab on Sublevel 2 and uses Paper Mode to slip thought the vent on the far right, they land in a changing stall where Peach used to put on the X-Naut suit. If they pull the curtain back, Mario and his partners become 8-bit. This lasts until they leave the room.
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