The hidden Elvish city ruled by Turgon in J.R.R. Tolkien’s First Age mythology was Gondolin.
Gondolin was founded by Turgon in a concealed valley surrounded by the Encircling Mountains. Its location was protected from Morgoth, the great enemy of the Elves, for many years. The city’s secrecy and beauty made it one of the most important strongholds of the Noldorin Elves.
Its story is told most fully in The Silmarillion and in Tolkien’s earlier writings. Gondolin eventually fell through betrayal by Maeglin, who revealed its location to Morgoth. The assault destroyed the city, but survivors escaped under the leadership of figures including Tuor and Idril.
Doriath and Nargothrond were also important First Age Elvish realms, which makes them plausible alternatives. Eregion belongs mainly to the Second Age and is associated with the making of the Rings of Power.