Castle Grayskull is the central location of the MOTU mythos. Shaped like an enormous skull jutting from a stony plateau, drawbridge hinged at the lower jaw, it sits on Eternia at the geographic and symbolic opposite end of the planet from Skeletor’s Snake Mountain. Inside it the Sorceress maintains a constant vigil; from inside it she communicates with the Heroic Warriors via her magical gifts.
The castle’s exact contents are described differently across continuities. In the original 1982 toy playset it includes a throne, a weapons rack, a dungeon, and a hidden lever-operated trapdoor. In the Filmation cartoon it is shown as a vast mystical space with corridors and chambers that don’t appear to occupy consistent physical space. In the 200x MYP series and subsequent continuities it is established as the resting place of King Grayskull and the source of the Power of Grayskull that becomes He-Man’s strength.
The castle’s central narrative function is to be both the prize Skeletor pursues and the source He-Man defends. Across nearly every continuity the Power Sword is shown as the key to its secrets in some form.
Catalogue source: heman.fandom.com (CC BY-SA 3.0).