Gondolin is the great city hidden in a valley in Beleriand in J.R.R. Tolkien’s legendarium. Its name means “Hidden Rock” in Tolkien’s constructed languages, and it is also called the Hidden City.
The city was founded by Turgon, an Elven king, during the First Age. It stood in a concealed valley surrounded by the Encircling Mountains, which helped protect it from Morgoth, the first Dark Lord. Gondolin remained hidden for centuries while other Elven kingdoms and strongholds fell.
Its location was eventually revealed through the treachery of Maeglin, a powerful Elf who betrayed the city after being promised power and Idril. Morgoth’s forces destroyed Gondolin, although Tuor, Idril, their son Eärendil, and other survivors escaped. Gondolin belongs to the First Age, so it is not the same place as Gondor, the later human kingdom in The Lord of the Rings.