Elemental Realms

The Elemental Realms, also known as the Evolved Realms, are a collections of Worlds that were hidden away after the Keyblade War had ended, by former Keyblade Master Yen Sid and Master Eraqus, as a response to the Darkness that emanated from the Worlds after the failed attempt to bring back the original Kingdom Hearts. Not much is known about the Elemental Realms other than that they don't have the same Keyholes as other Worlds, and that they are capable of being infested with Heartless, Nobodies, or Dream Eaters to a minor extent, although the most powerful of said beings are unable to appear in them.

Known Elemental Realms
There are many different Elemental Realms, however only a few have been discovered so far by travellers. This is the discovered Worlds:
 * Amestris, Land of Alchemy
 * Hogwarts, Castle of Magic
 * The Walls, Land of Titans