Gondolin was built in the hidden valley of Tumladen in Tolkien’s The Silmarillion.
Tumladen was a concealed valley in the Encircling Mountains of Beleriand during the First Age. King Turgon founded Gondolin there after receiving a vision that directed him toward a secret refuge.
The city was protected from the forces of Morgoth for centuries. Its hidden location allowed the Noldorin Elves to preserve a powerful kingdom, but the city was eventually betrayed by Maeglin, who revealed its location to Morgoth.
Gondolin’s fall is one of the great tragedies of Tolkien’s First Age mythology. The story is related in The Silmarillion and was also treated in expanded form in The Fall of Gondolin, a later publication edited by Christopher Tolkien.