The fictional city destroyed in J.R.R. Tolkien’s The Fall of Gondolin is Gondolin.
Gondolin is a hidden Elven city founded by Turgon during the First Age of Tolkien’s legendarium. It stands in a concealed valley and is protected from the forces of Morgoth for many years.
The city is eventually betrayed, allowing Morgoth’s armies to attack. Its fall is one of the major tragedies of the First Age, involving Tuor, Idril, Eärendil, and the Balrog known as Gothmog.
Tolkien began writing the story during or soon after the First World War. A version was published posthumously in 2018 as The Fall of Gondolin, edited by Christopher Tolkien and illustrated by Alan Lee.