The Thousand-Year Door is a role-playing video game (RPG) with other nontraditional RPG elements. The player controls a two-dimensional version of Mario and explores a variety of worlds designed to look like paper.[3] In these locales, he is tasked with retrieving seven Crystal Stars which involves Mario completing puzzles and defeating enemies to proceed.
In the overworld, the player can find items that can be used in and outside of combat. The effects of these items range from healing Mario or his partner to damaging the opponent.[4] Mario can also purchase "badges" from non-player characters (NPCs), find them hidden in the environment, or occasionally obtain them from defeated enemies; each badge requires a certain amount of Mario's badge points (BP) in order to be equipped.[5] When equipped, these badges can permanently enhance a particular skill or aspect, or, in some cases, give Mario new abilities.[6] Throughout the game, up to seven characters are permanently added to the player's party; one character is present alongside Mario at any given time and can be switched out at any point. Each character has a specialized skill, some of which are required to solve puzzles. These skills include activating switches and removing physical barriers. Mario is also "cursed" with abilities that allow him to fold into a boat or a paper airplane; these are performed when he stands on a special "activation panel".[2] During the interlude between the game's chapters, the player controls Peach in the X-Naut Fortress and Bowser in multiple side-scrolling levels based on the original Super Mario Bros.[3]