The Thousand-Year Door is a magical door that was designed to seal away the demon that destroyed Rogueport years ago. The door also serves as an enterance to the Palace of Shadow, a place where the demon's soul rested for thousands of years. The door was sealed after the demon was defeated by the four heroes with the use of the Crystal Stars.
In front of the door is a pedestal that Mario stood on to raise the magical map before the door. Here, the door gave the location of the next Crystal Star by illistrating it on the map. After finding all the stars, the door cracked open, opening the pathway to the Palace of Shadow. Here, the demon was resurected in the burial chamber but was stopped by Mario for planning to take over the world once more.