Castle Grayskull sits at the center of the known universe on the planet Eternia. It houses the very power that created the cosmos, it serves as home to the Sorceress and refuge to He-man and the Heroic warriors of Eternia. It is the center of many battles thru out the cartoon series and various comics/magazines as Skeletor and his evil warriors make attacks to claim the castle and the great power that rests within its walls.
Origin #1 (Early Mini-Comics & Other Publishing)
In the early publications, Castle Grayskull was known as the home of He-Man, who used his half of the Sword Of Power to unlock the drawbridge, like a key. It was also said that the mighty fortress was the home of the Masters Of The Universe.
Origin #2 (Filmation)
This origin of the MOTU mythos described Castle Grayskull as having many powerful secrets, that the Sorceress protected. It was the home of the Sorceress and He-Man was sworn to protect it from all evil. This origin is the begining of the idea of Prince Adam, who lived in the Royal Palace Of Eternos, was the "Secret Identity" of He-Man. He would use the magic Sword Of Power to change into He-Man. This version also revieles that the title of "Sorceress Of Grayskull", was a title that was passed down for generations.
Origin #3 (200X)
The castle was shown in the same way as the Filmation origin. But in this rendition, there was back history involved in it. The debut of a man named King Grayskull, who ruled the domain back during the First Ultimate Battleground. He was the ancestor of He-Man, who once weilded the Sword Of Power. It is also revieled that, Grayskull's wife, Veena, becomes the first Sorceress Of Grayskull.