Gondolin is the hidden Elven city ruled by Turgon in Tolkien’s First Age.
Turgon founded Gondolin after receiving guidance from the Vala Ulmo. The city was built in a concealed valley surrounded by mountains and became one of the great strongholds of the Noldor in Beleriand. Its secrecy protected it from Morgoth for many centuries.
Gondolin was eventually betrayed by Maeglin, who revealed its location to Morgoth. The city fell during a devastating assault involving Balrogs, dragons, and Orcs. Survivors included Tuor, Idril, and their son Eärendil, whose later story connects the First Age to the fate of Middle-earth.
Gondor is a much later kingdom in the south of Middle-earth, while Doriath and Nargothrond are other First Age realms. Their similar names can cause confusion, but Turgon’s hidden city is specifically Gondolin. The story is central to The Fall of Gondolin.