Unlockable stage

An unlockable stage is a stage in the  series that is not available at the start of the game (unlike a starter stage), but must be unlocked. Like an unlockable character, it is required to accomplish a particular in-game feat and once completed, a notice will be shown, telling the player that the stage has been unlocked.

In demo versions
These stages were made unlockable when first introduced in their respective demo versions: