Castle Grayskull is a fortress on the planet Eternia. It is home to the Sorceress who keeps the secrets of Grayskull protected and is the source of He-Man's powers. While the fortress has been redesigned in He-Man's various incarnations, the building always features a prominent skull in it's design. The castle is regularly threatened by Skeletor, who wants the castle's power for himself.
Though it never mentions who the architects were. the original origin of the castle was build as part of the kingdom of King Grayskull back in the early dark ages of eternian history, its been said at 1000 years old. The castle since the loss of its original owner has been taken care of by generations of women taking upon the mantle called the sorceress, the first was Queen Veena the wife of King Grayskull. many centuries have pass and the current caretaker is the Sorceress (Teela-na), Mother of Teela.
The castle itself lies in the lands owned by the eternian kingdom Eternos, which is control by King Randor, ancient descendant of King Grayskull. much of the history of Castle Grayskull faded into obscurity and legend as the Council of Elders took over as watchers of Eternia. The Castle sits in the northern region of the evergreen forest of the Light Hemisphere. It would be only after the appearance of He-man that the castle becomes a point of interest and a military focal point between the palace of Eternos and the variety of creatures that would seek to gain access to the castle.
Injustice vs. Masters of the Universe
In the series Injustice vs. Masters of the Universe, Castle Grayskull becomes the place where the Anti-Life Equation resides, found by He-Ro and guarded in place since the castle itself is the nexus of all realities and Eternia's equivalent to the Rock of Eternity from DC. Because of this, Darkseid decides to attack Eternia and the Castle of Grayskull in order to finally get their long-awaited power.
He-man and the Masters of the Universe (Filmation 1983)
During the original series, There was no origin of the castle stated, only just a legend that it would appear when all hope was lost. This series also does not mention what the secret of Castle Grayskull was, but that Skeletor was determined to capture, destroy, or get what ever secret there was inside. No real consistency was made with the castle as much of it was a plug to push the toy lines for the series.
Masters of the Universe Movie (1986)
The opening to the movie introduces us to the castle and its design. much of the design was a mixture of the Eternia playset from the he-man toy collection. it was given key point of focus but little was given in the way of showing. outside of opening intro, nothing is scene of the outside of the castle, The interior was showcased as a large hall with statues of that of elders, or ancients of the past. though the writers stated that much of the design was hard to create due to lack of background information. The main room itself contained drapes, and several pitfalls that seemed to have lead into part of the moat that surrounds the castle. The key factor in the movie that at a certain pinnacle of the moons height sends down periodical rays of energy which given to a person gives them god like status.
He-man and the Masters of the Universe (2002 Mike Young Productions)
Mattel decided to bring back and revamp the old Masters of the Universe franchise, this time characters and places were given deeper and more complex story lines. this true was said to be of Castle Grayskull. In the beginning and middle of the first season we are treated to the secret that was kept behind the walls from the first series. the second was the origins of the castle itself and its original ruler. the castle itself has seen and survived many enemies, the likes of which of Hordak, King Hiss and the Snakemen, and Skeletor and his evil warriors. much of the original continuity and designs tried to stay true to the original series.