Castle Grayskull is a great craggy skull shaped lair located on the planet Eternia. It serves as He-Man's base of operation, and also serves as the locus of his power.
It has been 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.
Castle Grayskull, which resembles a gigantic skull, is the source of the Power of Grayskull. Inside the Castle lives the Sorceress. Part falcon and part woman, she possesses the magical ability to fully change herself into a falcon. The Sorceress is among the most powerful magic-wielders on Eternia. She has limited use of the power held within Castle Grayskull, but only for the purpose of its protection. She also communicates telepathically with He-Man, and was the person responsible for granting Prince Adam the power to become He-Man.
As part of the very popular 80's toyline, a Castle Grayskull toy was released, to almost unanimous adoration. It had its share of gimmicks; an elevator, trap door, draw bridge etc.
Bob got mad today and stomped on his Castle Greyskull. Now he'll have to use his Ewok Village action playset, instead.