X-Naut Fortress

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 driving Magnus 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 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 is all connected by a series of hallways and two elevators. The deeper Mario or Peach go, the dimmer the lighting is.

Enemies

 * Elite X-Naut
 * Sir Grodus (Never actually fought) (Peach Intermissions only)
 * Magnus Von Grapple 2.0 (Chapter 7 Boss)
 * Mini-X-Yux
 * Mini-Yux
 * Mini-Z-Yux
 * X-Naut PhD
 * X-Naut
 * X-Yux
 * Yux
 * Z-Yux

Formations
For enemy formations in the X-Naut Fortress, see here.

Area tattles

 * "Who knew there's be such a weird building on the moon... It's kind of crazy, actually. I mean, it's got a big glass dome to keep all of the air from leaking out...What's that? Yeah, now that you mention it, I guess we were OK out there with no air to breathe...................................Yeah, I'd rather not think about that too much, I guess. Let's change the subject."
 * "This is the X-Naut Fortress. This seems to be the only way in or out. The stars look really nice through these windows, but I guess now's not the time..."
 * "This is a fortress corridor. An elevator connects it to the lower floors. I guess this fortress extends pretty far beneath the surface of the moon, huh? I mean, I hate Grodus, but I gotta give him some props for a pretty cool evil hideout."
 * "This room looks like it was meant to store something... It's booby-trapped, I think. Not to totally change the subject or anything, but how's your memory, Mario? If you have trouble recalling the full path, you oughta leave and reender."
 * "This looks like a storage room for valuables. The crane can grab stuff up there. It should work if you set a gear in place and press those switches in order... I'm sure you can handle it. It's not exactly moon-rocket science here."
 * "What's this room for, huh? I feel like I've seen that face somewhere before... Hmmm... Oh well, forget about it, I guess."
 * "This room looks like it was meant to store something... It's booby-trapped, I think. If you wander off the correct path, it looks like you'll take damage, don'tcha think? Rooms like this ALWAYS have a hint, though. It's like, a law. See anything like that?"
 * "This is Grodus's lab. He must have his researchers work on various potions here. I don't think we should touch any of this stuff, though... One wrong move and... KABOOOOOOOM! ...Know what I mean?"
 * "This looks like some kinda teleporter. Who knows where that thing would send us! Maybe that's what the X-Nauts used to go between the moon and the earth. I think we have to switch it on and then stand on the platform to use it."
 * "This looks like a lab for genetic experimentation...maybe Yux were made here. I guess they must have some pretty advanced technology if they can do stuff like that. Hey, by the way, Mario... You see up there? One of those ceiling panels is loose. Don't you have some super move or something that can get us up there?"
 * "This is a changing room. I guess even X-Nauts have to change out of their uniforms... But what do they look like under those suits? I'm...not gonna think about it. There's a ventilation duct in the ceiling at one end... That's not good for privacy!"
 * "This place is a bit overdone, huh? It just screams, 'Hey! I'm an evil mastermind!' I wonder if poor Princess Peach got dragged in here before?"
 * "What's this room for, huh? Is it some kind of office for the X-Naut boss or what? Maybe we should give it a once-over to make sure we don't miss anything. Boy, and what's with these tanks full of Nibbles? Jeepers, that creeps me out."
 * "This room looks like it was meant to store something... It's booby trapped, I think. If you stay on the tiles that were lit up, I think you'll be OK, Mario. Er... But you should totally go first, one way or the other..."
 * "There are three panels here. I guess that means we need three Card Keys, huh? There must be something pretty important behind this door to have such security..."
 * "This is the X-Naut factory. I think they make all kinds of stuff here. It's pretty complicated, what with the crane and the pipe, and all those gears... Maybe we should just take this room one bit at a time, don'tcha think?"
 * "This is where they built Magnus Von Grapple and Magnus Von Grapple 2.0. But there doesn't seem to be anything here now... Nope. Let's go, Mario."
 * "This is the computer room. This must be where Princess Peach met with TEC."
 * "Wonder what this room's for? There are beds and lockers, but we can't nap here. There's nothing else, really. The weird thing is, it kind of smells nice in here, huh? Why do you think that is?"
 * "This is a...shower room? I guess every evil-guy base of ultimate doom needs one... Just because you're evil doesn't meen you enjoy B.O., y'know what I mean?"

Trivia

 * 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 will 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 will become 8-bit. This lasts until they leave the room.
 * Grodus' room contains a tank filled with Nibbles. This is the only place where the player can see their entire body.
 * If players have Vivian out when they first enter the fortress, Vivian does not seem to recognize it, despite being inside the fortress before.
 * A single Elite X-Naut hides in the shower room on Sublevel 4 while the fortress is active.
 * This is the only Paper Mario game (or Mario game for that matter) that the main antagonist's base of operations is not the final area of exploration.