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The Castle Key is an item found in the Paper Mario series. Castle Keys are either stored in chests or held by someone.
In Paper Mario, Castle Keys are found in Tubba Blubba's Castle, Peach's Castle, and Bowser's Castle. There are three Castle Keys in Tubba Blubba's Castle. The first is found in the basement. Mario goes left from the entrance, and takes a path in the north to access the north area of the basement. There is a yellow Boo in the basement and tells Mario about Tubba Blubba. This key allows Mario to access the right area of the castle. The second one is found in a spiky room on the second floor. Mario should hide with Bow to avoid the spikes. The third one is found in a room before Tubba Blubba's living room in the highest place. The room is guarded by many Clubbas.
Princess Peach also get Castle Keys when she sneaks out with Twink. The first Castle Key is given by Gourmet Guy in Chapter 4. This key opens the door to the first floor where Princess Peach can bake a cake. The second Castle Key is given by a Koopatrol in Chapter 6. The Koopatrol asks Princess Peach, who disguises as another Koopatrol, to find a Clubba outside the castle. This key opens the door to the outside, but Princess Peach still has no way to descend to the earth.
Castle Keys are also found in Bowser's Castle, where there are five. The first one is held by a Koopatrol on the sideway near the entrance. Mario must defeat it to open the entrance door. The second is found in the lava area to the next. Mario should clear the puzzles there to cool down the lava. After that he can use the key to open the door to make progress. The third is in the water area, which also involves puzzle-solving. The fourth is found in the maze area, where the key is hidden at the end of a fork road. The fifth is in a platform area to the next. Mario should use Kooper or Bombette to lift the platforms that allow him to go to the high place and get the key. There are also two optional Prison Keys in Bowser's Castle.
In Paper Mario: The Thousand-Year Door, there are four Castle Keys in Hooktail Castle. The first one is on a ledge near the flippable switches, and it opens the door to the right. The second is behind the cells on the first floor. Mario and co goes back after getting the Paper curse to get it to access the second floor. The third is in the storage room on the third floor, where Mario encountered Ms. Mowz. It opens the door to the right. The fourth is in the right of the third floor. It opens the first door on the fourth floor, to the outside.
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