The First Age city hidden in a valley and ruled by Turgon in J.R.R. Tolkien’s legendarium was Gondolin.
Gondolin was a secret Elven city in the First Age, founded by Turgon of the Noldor. It stood in a concealed valley surrounded by mountains, and its location was protected from the forces of Morgoth for many years. The city’s secrecy was one of its greatest strengths, but its existence was eventually betrayed.
The fall of Gondolin is one of Tolkien’s oldest and most important legends. Maeglin reveals the city’s location to Morgoth, whose armies attack it. Tuor, Idril, and their son Eärendil escape through a secret way, while many of Gondolin’s inhabitants are killed. Gondolin should not be confused with Minas Tirith, the later city in Beleriand, or the Third Age city of the same name in Gondor.