Fortress Key

From the Super Mario Wiki

Fortress Keys are used to open doors in the Koopa Bros. Fortress in Paper Mario. There's four throughout the area, and each need to be used to progress. The Koopa Bros. hid some keys behind bars, and left some to be guarded by enemies.

The keys can only be used in the Koopa Bros. Fortress.

Keys
Black Key | Blue Key | Card Key | Castle Bleck Key | Elevator Key | Fortress Key | House Key | Key | Odd Key | Palace Key | Pedestal Key | Random House Key | Red Key | Ruins Key | Shop Key | Skeleton Key | Station Key | Steeple Key | Storage Key | Storeroom Key | Warehouse Key | Yakkey