In the series, it is the target of numerous attacks by Skeletor, Hordak and the Snake Men, all of whom believe that the secrets inside will allow them to conquer Eternia and become the titular masters of the universe.
Centuries ago, during the Great Wars, Castle Grayskull was first built as the temporary seat of the legitimate government of Eternia when the Royal Palace in the traditional capital of Eternos was occupied by King Hiss and the Snake Men.
During the Great Wars and until the reign of King Grayskull, Castle Grayskull was also a center of religion, particularly the worship of Eternia's Goddess.
More recently, Castle Grayskull was the home of the titular Sorceress of Castle Grayskull, with King Grayskull's wife Queen Veena being the first Sorceress. Veena was succeeded by Kuduk Ungol, who in turn was succeeded by Teela Na, the current Sorceress of Grayskull.
In the original series of pack-in minicomics Castle Grayskull features prominently. It is described as built by unknown beings before the Great War that wiped the old technology of Eternia. Thus, it's one of the last remnants of the powerful ancient Eternians. Skeletor seeks both halves of the Power Sword that is the key to Grayskull and gives the right to be king of the castle, and with its power, of Eternia and the whole universe.
In the original television series, Castle Grayskull is a legendary/mythical location situated on a pedestal of bones jutting out of an abyss in a barren plain just on the edge of the Evergreen Forest. It serves as home to The Sorceress, and 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 Main Chamber (incl. the Walkway of the Elders); Throne Room; the Hall of He-Man; the Portal Chamber; a laboratory; Mirror Room; the Sorceress's den and bed chamber (right tower), and "The Gallery Of Living Pictures;" this last was employed in the VHS home video "The Greatest Adventures Of All!" to show what the Sorceress called the greatest adventures of the Masters Of The Universe, especially He-Man himself. 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 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 phenomeneon 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 Portal Chamber, that serve as portals to various locations on Eternia, including Snake Mountain, and at least one that leads to Etheria.
The castle's purpose and history are significantly expanded upon in the 2002 remake, as are the variety of locations shown in the castle. The most significant of the expansions made is the addition of a large Crystal Chamber, accessible only by a secret "door" in the floor of a secluded chamber. Within this chamber is the Orb of Power, within which lies the combined powers of the Elders. Prior to its bestowal upon Prince Adam, the Sword of Power was also stored in the depths of the Crystal Chamber. Other locations within the castle include the Throne Room; a massive library; a room that houses an enormous, mystical mirror used to view both the past and present; an underground colosseum; a secret chamber devoted to King Grayskull; the area behind the eyesockets of the castle's fascade; and, numerous corridors that are draped in illusions to confound invaders. Once made of white marble, and at the center of King Grayskull's bustling kingdom, the castle fell into a state of disrepair following a devastating battle with the forces of Hordak. Over countless years, the castle was almost entirely forgotten by the populace. Its true purpose, and the secrets within, are now known to only a very select few. Like the Filmation version, it is surrounded by a seemingly bottomless abyss.
1987 Live action movie
In the 1987 live action movie, Castle Grayskull is finally captured by Skeletor's forces. The first image we have of its interior is as Skeletor, victorious at last, makes the long march across the castle's Throne Room.
The live action depiction of the castle is radically different from that in any of the other media.
The throne resides in the posterior section of the room just beyond the section's seemingly incomplete marble foundation which rests atop pillars and framework that extend from a deep abyss. This foundation is formed in such a way that there are four large holes of sizes that altogether occupy most of the floor's area leaving a few wide catwalk walkways. The large posterior section of the room is lined with several tall statues of elders along each side whereas the much thinner, less tall, anterior section of the room is an open hallway lined with columns along each side.
Near the throne, Skeletor has trapped the Sorceress within an energy field, which, little by little, transfers her power to him. The castle serves as Skeletor's main base of operations throughout the movie. According to the film, it houses a giant circular portal called "the Great Eye of the Galaxy", which only opens when Eternia's moon reaches its zenith, granting whoever stands before it god-like power. It is never explicitly stated, though visuals strongly indicate, that for this to happen the Sword of Grayskull must be inserted into a small column near the portal.
The opening narration implies that Castle Grayskull itself is at the center of the universe. It is also stated that the castle lies at "the border between the light and the dark", although the meaning of this is never expanded on.
The architectural style of the castle in the film is very different from the original depictions of the castle. The exterior is rarely seen, and much of the action relating to the castle takes place within the castle. The marble surfaces are smooth, and of a golden tan color. The place has a clean, well lit, high-tech appearance in some respects. This contrasts with the ancient green and grey, cyclopean masonry and stonework of the original castle.
Another difference is that there seems to be an actual city surrounding the castle. The city is never named. This depiction would seem to imply that the writers merged castle Grayskull and the Royal Palace/the city of Eternos.
Many of the prominent features seen in the animated series, such as the jawbridge and the various facilities of the castle are not mentioned in the film.
In the 2002 remake of He-Man, it is revealed that Castle Grayskull was once home to King Grayskull, an ancestor of Prince Adam, who died defending his kingdom and Eternia as a whole from Hordak. Upon his death, King Grayskull transferred his power into his sword that now forms the basis of He-Man's power. His wife Veena became the first Sorceress, and his advisers became the Council of Elders. He also had an enormous green lion as a mount, bearing the same armor as Battle Cat, and is presumably where Battle Cat gains his powers.
- Castle Grayskull appears at the beginning of the transformation sequences of the He-Man spin-off series She-Ra; near the end The Crystal Castle appears in its place.
Castle Grayskull was made as a playset for both the 1980s and 2002 Masters of the Universe toy lines. The 1980s incarnation was one of the more popular Christmas presents for pre-adolescent boys in the 1980s. The price point was generally within reach of the parents of a child in the target age range for the toy line. The castle was also user-friendly, possessing many settings and functions suitable for the 5" action figures it was built to complement, as well as being small enough to be put away at the end of a play session.