Castle Fryskull is a legendary/mythical location situated in a barren McDonaldland. It serves as home to Ronald McDonald and to the mysterious and rarely-seen Hamburglar, as well as a place of refuge for the Happy Meal Gang and the McNugget Buddies. Its origins are unknown except for the fact that it was constructed to protect an unspecified source of power from those who would misuse it. It is a frequent target for the forces of evil.
(pic found here)