Gondolin was the hidden city ruled by Turgon in J.R.R. Tolkien’s The Silmarillion.
Turgon, an Elven king of the Noldor, founded Gondolin in a concealed valley surrounded by the Encircling Mountains. The city’s location was protected from Morgoth, the first Dark Lord, and its people preserved an important Elvish kingdom during the First Age. Gondolin’s secrecy made it one of the strongest remaining centers of resistance.
The city was eventually betrayed by Maeglin, who revealed its location to Morgoth after being captured and offered power. Morgoth’s forces destroyed Gondolin, although Tuor, Idril, and their son Eärendil escaped with other survivors. Gondolin is different from Nargothrond, another hidden Noldorin realm, and from Doriath, the forest kingdom associated with Thingol and Melian.