The City of the Ancients was a location on Eternia seen in He-Man and the Masters of the Universe.
The City of the Ancients was located deep within the jungles and was considered lost for many years. A golden exterior featured the image of it's last known ruler, Demos prominently.
Deep within the bowels of the main castle were kept the Masks of Power, in which the spirits of Demos and his wife, Tyrella waited to be unleashed. The cavern also had a giant protector in the form of Aroo.
Prince Adam was the one to discover the location of the lost city, when on an archaeological search with Man-At-Arms and Orko. Unfortunately, Skeletor used a a Robot Bird to spy on them and sent two of his newst minions, Aran and Oona to fetch the masks of Power for him. Once they got their hands on the masks however, they felt compelled to put them on, and were instantly taken over body and soul by Demos and Tyrella. They left the Heroic Warriors to grapple with Aroo while they left to recover the fabled Sword of the Ancients.
In a cruel twist of fate, the powerful Sword of the Ancients was not kept in the City of the Ancients, but was burried deep inside Snake Mountain instead.