Castle Grayskull was conceived as the central location for the battle between the forces of good and evil. The original playset was released in 1982, and all other depictions of the castle originate from it. It is a large hollow green castle, consisting of two halves connected by a hinge. The toy is designed to open for play, and close for storage. A carrying handle is molded into the top of the castle. A large skull decorates the front, with a hinged "Jawbridge" allowing access to the interior, through the "mouth" opening. The interior has a throne room, elevator, and a trap door. Multiple accessories are also included - a ladder, removable flag pole, turret cannon, weapons rack with 9 weapons, and a "battle trainer" device,
The original concept art and prototype show a large swamp surrounding the castle, which was going to be printed on a playmat that would be positioned beneath the playset. This moat playmat was not included with the finished toy, in order to keep costs down.
The origin of Castle Grayskull was related in the read-along book "Castle Grayskull" as once being the beautiful "Hall of Wisdom," the "center of Eternian culture and a storehouse of all knowledge of the universe." The Hall was the meeting place of the Council of Elders. One day, the elders saw a vision of a beautiful woman dressed in snake armor (note that this description matches early versions of Teela AND the Sorceress, see for example, making an intended identity with the Sorceress very possible) who warned of future danger and also the coming of He-Man. The elders concentrated all of their power into a magical orb. The elders then magically transformed the Hall of Wisdom into Castle Grayskull in order to frighten away intruders and protect the orb. This was to have all taken place several centuries before the coming of He-Man. The Castle was then largely forgotten until Man-At-Arms eventually led Prince Adam to Castle Grayskull, where Prince Adam became He-Man using the Sword of Power.
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 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; Main Chamber (including the Walkway of the Elders); the Hall of He-Man; the Portal Chamber (in the skull top); a laboratory (basement); Mirror Room (basement); the Sorceress' den (where she studies and practices spells; in the right tower); and, the Sorceress' bed chamber. In the episode "Double Trouble", an evil doppelganger of He-Man's friend Kol Darr 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 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 series, 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 (trophy room); a massive library (blue room); 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 facade; 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, and is backed by a sheer cliff. This abyss was revealed to have been created during the final confrontation between King Grayskull and Hordak.
In the 1987 live action movie, Castle Grayskull is finally captured by Skeletor's forces. The first image of it seen is of its interior as Skeletor, victorious at last, makes the long march across the castle's Throne Room. 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.
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.
In the film Hot Fuzz, police officer Danny Butterman (played by Nick Frost) says "By the power of Greyskull!" when shown the stockpile of weapons collected by a local farmer. This line is later repeated by Sgt. Nicholas Angel, after seeing Danny's collection of action movie DVDs.
In the film Green Lantern, Hal Jordan attempts to transform, and says "By the power of Grayskull!" before he finds the real transformation sentence implanted in his memory.
In the film Kickin' It Old Skool, the lead character, Justin, played by Jamie Kennedy, shouts "By the power of Grayskull!" as he sees the available information on the Internet.
German epic death metal band the Grailknights released the album Return to Castle Grailskull, picturing them on the album cover in front of "Castle Grailskull" (resembling the sculpt used in the 2002 MOTU toyline). In the first video clip from that album, "Moonlit Masquerade", they perform on a CGI-generated Castle Grayskull. The band's live shows include characters who resemble Skeletor and other MOTU figures.
At WrestleMania XXVIII, professional wrestler Triple H made his way to the ring through a structure resembling Castle Grayskull.
The 23rd Regiment Armory in Brooklyn, because of its Romanesque Revival design style and castle appearance since the first run of the He-Man animated series, has been given the nickname "Castle GreySkull".
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 castle was user-friendly, possessing many settings and functions suitable for the 5" action figures it was built to complement.
Castle Grayskull was released by Mattel for inclusion in their Masters of the Universe Classics action figure line (mid to late November 2013), and a new character, named Castle Grayskullman, described as the "Heroic Embodiment of Castle Grayskull" was created by Daniel Benedict, and was the winner of the 30th Anniversary "Create-A-Character" Contest. The figure of Castle Grayskullman was released in November 2012.