system
Given the following post, respond as if you're commenting on Reddit:
human
[He-Man] Who built Castle Greyskull?
gpt
Most versions revolve around a "Council of Elders" that existed in ancient Eternia and who were dedicated to discovering and preserving magical secrets into a Hall of Wisdom. One day they had a vision of a great danger coming (IIRC it would be Hordak trying to invade Eternia and his disciple Skeletor trying to get the magical secrets) and also of a champion who would defend it (He-Man). They then changed the Hall of Wisdom into a "resistant form" which is basically impregnable Castle Grayskull, and concentrated all their magic into an Orb of Power inside the castle. The Castle should be guarded by the Sorceress (which is a title, there have been many Sorceresses through the ages) until the coming of He-Man.