The Elvish city ruled by Turgon in J.R.R. Tolkien’s First Age stories is Gondolin.
Gondolin was a hidden city founded by Turgon in a valley surrounded by the Encircling Mountains. Its location was concealed from Morgoth, the great enemy of the Elves and the other peoples of Beleriand.
The city became one of the major realms of the Noldorin Elves. Its secrecy lasted for centuries, but Maeglin eventually revealed its location to Morgoth after being captured and corrupted. Gondolin was then attacked and destroyed during the First Age.
Gondolin should not be confused with Doriath, the woodland kingdom of Thingol, or Nargothrond, the underground realm associated with Finrod and later Turin. Its story is told in The Silmarillion and in unfinished texts edited by Christopher Tolkien.