Elven city in Beleriand, featured in Tolkien's The Silmarillion. It was founded by Turgon in line with a dream to recreate Tirion and also to be hidden from Morgoth. Anyone who found their way to the city was forbidden to leave, and its location was not known to outsiders (except Ulmo who doesn't count). It was hidden in a glade called Amon Gwareth surrounded by mountains, near the start of the river Sirion, in the Ered Gorgoroth mountains.
The elves of Gondolin left the place only once, to fight at the Nirnaeth Arnoediad. After their defeat, they retreated, covered by Hurin and his army, and returned to Gondolin.
Hurin and Huor found their way to Gondolin but were allowed to leave. An elf-woman related to Turgon also left, and as a result the dark-elf Eol and his son Maeglin found their way to Gondolin. Eol was slain after he killed the elf-woman because Turgon wouldn't let him leave. Maeglin stayed but was driven to distraction because he fancied Idril but they were cousins or something, and then she married Tuor son of Huor when he found his way to Gondolin, and he was a human not an elf.
Finally betrayed by Hurin who, in a state of madness after being freed from Angband, cried out at the gates, and Maeglin, who, on a secret trip out of the mountains, was captured and threatened/bribed into helping Morgoth's forces enter. The city was sacked and burned, the elves slaughtered and only a few escaped.
Turgon was slain in a battle with Gothmog the Balrog who was also slain. Tuor and Idril were among those who escaped.
Gondolin lasted longer than any other elf kingdom.
The name means "song of stone" in Sindarin Elven.