Castle Grayskull is a legendary/mythical location situated in a barren wilderness. It serves as home to The Sorceress and to the mysterious and rarely-seen Spirit of Grayskull, as well as a place of refuge for He-Man and all those considered his friends. 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.
Although it is mostly viewed from the outside, there are a few notable internal locations: the Throne Room; the Hall of He-Man; the Travel Corridor Chamber; a laboratory; a room used by the Sorceress for studying and practicing spells; and, the Sorceress' bedroom. In the episode "Double Trouble", an evil doppleganger of He-Man's friend Koldar also comes across the chamber that houses the Secrets of Castle Grayskull. It is a room with two doors, between which sits the Guardian, one of which houses the secrets, the other one is only destruction.
The castle is surrounded by a bottomless abyss, which doubles as a moat in at least one episode, and a Jawbridge (a drawbridge formed by the lowerjaw of the castle's "mouth") is used to come across it. When Prince Adam transforms into He-Man, sparks shoot up from the abyss towards He-Man's sword. This phenomenon is seen in the episode "Into the Abyss", when Teela falls into the abyss and witnesses this spectacle from a unique vantage point. In the same episode, He-Man states that the abyss actually leads to the center of Eternia.
There are a number of magical doors inside the castle, most of them in the Travel Corridor Chamber, that serve as portals to various locations on Eternia, including Snake Mountain, and at least one that leads to Etheria.