Key
There are dozens of different keys for castles and fortresses all across the Mushroom World.
Super Mario Bros. 2 is the first Mario game with keys and locked doors. Phantos will always be guarding keys, so unless you drop the key or unlock a door, a Phanto will be chasing you. Keys are always in view; never under a Vegetable.
In Super Mario World, the keyholes would turn into a gigantic portal when a key came into contact with it.
In Super Mario World 2: Yoshi's Island the keys unlocked doors, but when a boss was defeated a large key would appear and act the same as ones in Super Mario World.
There's many key designs in Paper Mario and Paper Mario: The Thousand Year Door but they only open specific doors. There's Castle Keys for every castle and fortress, and various others keys in differnt locations.
