Gondolin is the First Age city defended by Turgon in J.R.R. Tolkien's The Silmarillion.
Turgon, a king of the Noldor, establishes Gondolin in a hidden valley surrounded by the Encircling Mountains. The city's location is concealed from Morgoth for centuries, allowing its Elves to preserve a powerful realm during the wars of Beleriand.
Gondolin eventually falls after Maeglin betrays its location to Morgoth. The attack brings destruction to the city, but Tuor, Idril, and their son Eärendil escape with other survivors. Eärendil later becomes one of the most important figures in Tolkien's mythology.
Gondolin is one of several major hidden or fortified Elven realms in the First Age. Nargothrond is associated with Finrod and underground halls, while Menegroth is the residence of Thingol in Doriath. These similar settings make the names easy to mix up, but Turgon's kingdom is Gondolin.