A Steeple Key is a key that unlocks a door in the Creepy Steeple in Paper Mario: The Thousand-Year Door. There's two versions, one that works in the regular building, or the one for the basement.
The white key unlocks the staircase leading up to Doopliss. To get the key you must first release the Boos stored in the box and guess the correct amount of Boos that were in there. Afterwards when you return to the main area of the castle ignore the boos and go out through the bottom door and go over to hit the red switch once. Go inside and up the stairs. Walk forward and use Koops to get the key.
The purple key unlocks the room Doopliss's parrot is in. To get it you need to have done the Doopliss battle, afterward you become Shadow Mario for a bit, in which time after heading back to Twilight Town, Vivian will join the party after you find the broken Superbombbomb for her. You go back to Creepy Steeple and go down the well and use Vivian's ability to hide underground while the glass structure slides back down. Then you ground pound and head left to the room where the parrot is to get "P" and the purple steeple key which unlocks the room.
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