Trapdoors[1] are doors that lead to special rooms in the game Wario World. Wooden trapdoors[2] feature small obstacle courses or puzzles, which are usually underground but are always in a contained space. They are distinguishable from steel trapdoors[2] by the "X" shape of bandages on them. Wario can enter them by Ground Pounding them.
Unlike wooden trapdoors, which can be entered with a Ground Pound, steel trapdoors require a Piledriver to get through. Once inside, Wario finds himself in an expansive obstacle course in the sky, with no solid ground at all. Falling off the floating platforms forces him to start over, but Wario does not lose any health. Steel trapdoors are typically platforming challenges. There are no steel trapdoors in Greenhorn Forest.
Wooden and steel trapdoors reward the player with a red diamond, and possibly a Gold Statue piece. Other possible rewards for clearing trapdoors are Spritelings or a wooden chest. As the game progresses, the trapdoors become more difficult, some even placing some rewards halfway through instead of at the end.
Goal trapdoors[2] are located at the end of levels and lead to boss battles. Stone Doohickeys sit on top of these trapdoors, and the only way to remove them is to bring a certain number of red diamonds to them. The number of diamonds necessary to remove a Stone Doohickey increases as Wario progresses through the game.