From the Super Mario Wiki
The X-Naut Fortress is Sir Grodus' main base of operations which is located on the Moon and the home to the Secret Society of X-Nauts in Paper Mario: The Thousand-Year Door. This place is also where Princess Peach was taken to. The last Crystal Star, the Crystal Star, is held there and Mario and his friends had to use the cannon in Fahr Outpost to reach this place and fight Lord Crump (while driving Magnus 2.0) to get the Star. Grodus traveled between the Mushroom World 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 Lord Crump/Magnus von Grapple 2.0, he gets the seventh Crystal Star. After that, the fortreses 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's all connected by a series of hallways and two elevators. The deeper Mario or Peach go, the dimmer the lighting is.
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