Forest of Illusion is a location in Super Mario World that is ruled by Roy Koopa. It has 1 Fortress, and 1 Castle. It is the location of the Blue Switch Palace. It is, as the name of the world insinuates, an illusion, because there is a secret exit in almost every level.
The soundtrack for that map sounds different in the original Super Mario World and its remake, Super Mario Advance 2: Super Mario World.