The X-Naut Fortress is Grodus' main base of operations which is located on the Moon and the home to the X-Naut Legion in Paper Mario: The Thousand-Year Door. This is also where Princess Peach was taken to. The last Crystal Star found by Mario, the Crystal Star is held here. Mario and his friends had to use the cannon in Fahr Outpost to reach the moon and then had to fight Lord Crump (who was driving Magnus Von Grapple 2.0) to get the Star. Grodus travelled 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 6th Crystal Star, Sir 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 7th Crystal Star. After that, the fortress explodes and Mario is unable to return until the Shadow Queen has been defeated.
The X-Naut Fortress has a lot of floors and sub-floors.