Located on Skydancer Mountain, the Crystal Castle was the most magical site on all of Etheria. According to an ancient legend, the Crystal Castle contained all of the secrets from the time of the First Ones -- the founders of the planet Etheria. However, the Castle was protected and kept secret by a spirit keeper called Light Hope, a benevolent spirit aiding the forces of good in their times of need.
The Crystal Castle's location was known only to a rare few. Among them was Agho, the King of the Trolls, who met Light Hope and promised that he would not reveal the location of the Castle to anyone. This vow was challenged in later years during an eclipse of Etheria's moons. Castle Bright Moon was under attack by the Evil Horde, assisted by a powerful magical attack from Shadow Weaver. The eclipse would cause Bright Moon's power to wane, thereby enabling its destruction and, in turn, potentially ending The Great Rebellion. To help prevent this, She-Ra went on a quest to find Agho, locate the Crystal Castle, and learn its ancient secrets.
Though Agho kept his promise and did not tell She-Ra the location of the castle, he did hint at it with a riddle that said: "it will be where you most want it to be. In plain sight, but never seen." She-Ra deduced from this that the Castle rested on top of Skydancer. Once there, she met with Light Hope who told her of the Great Clock resting at the center of Etheria, which could save Castle Bright Moon by speeding up time. Light Hope then asked that She-Ra also keep Crystal Castle a secret, but allowed her to visit the site as her own personal haven. She-Ra therefore became protector of the Castle.