The great hidden city of Gondolin was destroyed by the forces of Morgoth in the First Age. In Maeglin's treachery, he confessed the secret location of Gondolin after being tortured by Morgoth and promised the hand of Idril.

Many Elves perished in the city's fall. King Turgon, the King of Gondolin was one of them. But, many also escaped through a secret way out of Gondolin that had long been prepared by Idril. Among these who escaped the Fall of Gondolin were Tuor, Idril, and their son EƤrendil.
Encyclopedia entry originally written by Lady~Eowyn