Which Tolkien story features the character Tuor and the hidden city Gondolin?

The Tolkien story featuring Tuor and the hidden city Gondolin is The Fall of Gondolin.

The Fall of Gondolin tells how Tuor reaches the secret Elvish city of Gondolin and warns its king, Turgon, about the danger posed by Morgoth. Gondolin is one of the great hidden kingdoms of the First Age, protected for many years from the enemy.

Tolkien began developing the tale during the First World War, and it became one of the earliest stories in what later became his legendarium. A standalone edited edition, prepared by Christopher Tolkien, was published in 2018 with illustrations by Alan Lee.

The story is different from The Children of Húrin, which centers on Túrin Turambar, and from the tale of Beren and Lúthien, which follows the romance and quest of its two title characters. All three belong to Tolkien’s First Age mythology.