Gondolin is the Hidden City in J.R.R. Tolkien’s The Silmarillion.
Gondolin was an Elven city of the First Age, founded by Turgon in a concealed valley surrounded by mountains. Its location was protected by secrecy, and the city became one of the strongest and most beautiful realms of the Noldor in Beleriand. Its name is associated with the Sindarin language and is commonly translated as “Hidden Rock.”
The city’s secrecy eventually failed through the betrayal of Maeglin, who revealed its location to Morgoth. Gondolin was then attacked and destroyed by Morgoth’s forces. Tuor reached the city after receiving a warning from Ulmo, and he later married Idril, Turgon’s daughter. Their son was Eärendil, an important figure in Tolkien’s mythology.
Gondolin should not be confused with Gondor, the later kingdom of Men in The Lord of the Rings. Nargothrond was another major hidden realm, but it was a separate underground city.