The hidden city within the Encircling Mountains in J.R.R. Tolkien’s The Silmarillion is Gondolin.
Gondolin was an Elven city founded by Turgon during the First Age of Middle-earth. It stood in a concealed valley surrounded by mountains, and its location was kept secret from Morgoth, the great enemy. The city became one of the major strongholds of the Elves in Beleriand.
Its secrecy eventually failed. Maeglin, a member of Gondolin’s royal family, betrayed the city’s location to Morgoth after being promised power. Gondolin was then attacked and destroyed by Morgoth’s forces. Tuor, a Man, reached the city and married Idril, Turgon’s daughter; their son was Eärendil, an important figure in Tolkien’s mythology.
Gondolin is distinct from Doriath and Nargothrond, other Elven realms of the First Age. Angband, meanwhile, was Morgoth’s fortress rather than an Elven city. The story of Gondolin appeared in several forms during Tolkien’s lifetime and was later published in expanded form by Christopher Tolkien.