The fictional city in J.R.R. Tolkien’s tale “The Fall of Gondolin” is Gondolin.
Gondolin is a hidden Elven city in the First Age of Tolkien’s legendarium. It was founded by Turgon in a concealed valley surrounded by mountains, and its location was protected from Morgoth, the great enemy of the Elves and Men. The city’s secrecy eventually failed, leading to its destruction by Morgoth’s forces.
The story was one of Tolkien’s earliest major tales of Middle-earth. He began writing it during or shortly after the First World War, and versions of it appeared in several stages of his developing mythology. A standalone edition edited by Christopher Tolkien was published in 2018.
Gondolin is easily confused with Gondor because the names look similar. Gondor is a later kingdom in the histories of The Lord of the Rings, while Gondolin belongs to the much earlier First Age and is chiefly associated with the Elves.