The Thousand-Year Door

Not to be confused with the Nintendo GameCube game, Paper Mario: The Thousand-Year Door. The Thousand-Year Door is a massive door in Paper Mario: The Thousand-Year Door that was said to have a treasure behind it. Generations of locals have created myths about what's behind it, and nobody knew that it turned out to be the entrance of Palace of Shadow. The only person that knows Rogueport's true history is Grifty.

The Thousand-Year Door, like the Crystal Stars and the Palace of Shadow it leads to, was created to hold back a horrifying demon that had destroyed the town that once stood within Rogueport's boundaries. The Four Heroes' designing made it so that the Crystal Stars were required to open this door, although their seal on the door would only last for a thousand years and was therefore continually weakening before the events of Paper Mario: The Thousand-Year Door. When Mario placed all seven Crystal Stars before the door, the dark power caused the door and walls around it to crack. A vortex spewed out, and for the first time in a millennia, the Shadow Queen's stolen home was open. During the events of Paper Mario: The Thousand-Year Door, the so-called "treasure" behind the Door was eventually thought to be the Shadow Queen herself, but the ending actually showed that there was indeed a treasure of sorts behind it: a near-useless Dried Shroom, which could have been a much more powerful mushroom in the past but dried to a Dried Shroom after being held captive for 1 millennium.

Because of legends and probably a bad vibe from the area, very few people have even dared to touch the door. Interestingly enough, the door seems thin because of its height, but in comparison to Mario, it should be several feet thick.

During the fight with the Shadow Queen, the pedestal in front of the Door is shown to be directly underneath the center of Rogueport (the gallows). During the long cutscene halfway through the battle, the player sees, around the gallows, the pedestal's lights, as well as each of the Crystal Stars.

Trivia

 * A Sammer Guy in Super Paper Mario is called "Thousand-Year Roar", referencing either the Thousand-Year Door itself or Paper Mario: The Thousand-Year Door 's subtitle.
 * Despite the fact that they lacked the Crystal Stars, Grodus and his league of X-Nauts were apparently able to enter the Thousand-Year Door before Mario and friends did, or so said "Professor Frankly" (actually Doopliss in disguise). It is possible that they went into the Thousand-Year Door while it was weakened, or that they sneaked in alongside the Shadow Sirens (who had simply used Mario and their rushing as a device to get into the Palace of Shadow themselves) and had therefore actually not been in before Mario.
 * In the final scene in which the player takes control of Bowser, Kammy Koopa immediately recognizes the Thousand-Year Door and its rumored treasure when Bowser asks what it is. It is unknown what the source of her knowledge of the door was; Kammy interestingly seems to be the only non-Rogueport resident who knew of the Thousand-Year Door before the events of Paper Mario: The Thousand-Year Door, though it is possible that she learned of it on her Rogueport-spanning travels with Bowser.