In the writings of J.R.R. Tolkien, the "Fall of Gondolin" is the name of one of the original Lost Tales which formed the basis for a section in his later work, The Silmarillion.
"The Fall of Gondolin" tells of the founding of the Elven city of Gondolin (built in secret by Turgon and his people), of the arrival of Tuor, a prince of the Edain, of the betrayal of the city to Morgoth by Turgon's nephew Maeglin, and of its subsequent destruction by Morgoth's armies. It also relates the flight of the fugitives to the Havens of Sirion, the wedding of Tuor and Idril, as well as the childhood of Eärendil.
Tolkien began writing the story that would become "The Fall of Gondolin" in 1917 in an army barracks on the back of a sheet of military marching music. It is more or less the first traceable story he wrote down on paper about his legendarium.
Tolkien was constantly revising his First Age stories; however, the narrative he wrote in 1917, published posthumously in the Book of Lost Tales, remains the only full account of the fall of the city. The narrative in The Silmarillion was the result of the editing by his son Christopher using that story (minus some elements all too obviously evocative of World War I warfare) and compressed versions from the different versions of the Annals and Quentas as various sources. The later Quenta Silmarillion and the Grey Annals of Beleriand, the main sources for much of the published Silmarillion, both stop before the beginning of the Tuor story.