Game Boy World is a world that can be accessed by going into the well after you've gone to the spirit world, in that room will be several characters and to the lower right of the room will be two plants.
When you walk on the lower plant, it takes you to this place modelled in the classic Game Boy format.
You will find that you can't get into all of the houses, although at least three have something interesting to offer. Stepping on a swirly tile in one of the houses takes you to an old factorial maze;
While another house leads to a place where you can see Koraiyn at it's home.
Another thing that should be noted is that sometimes you can encounter a 'Missingno' in one of the other houses that you can go into, interacting it will trigger an event.