Gondolin was founded by King Turgon in the First Age. It was originally named Ondolindë. According to The Silmarillion, the Vala Ulmo, the Lord of Waters, revealed the location of the Vale of Tumladen to Turgon in a dream. Under this divine guidance, Turgon travelled from his kingdom in Nevrast and found the vale. Within the Echoriath, the Encircling Mountains, lay a round level plain with sheer walls on all sides and a ravine and tunnel leading out to the southwest known as the Hidden Way. In the middle of the vale there was a steep hill which was called Amon Gwareth, the "Hill of Watch". There Turgon decided to found a city, designed after the city of Tirion in Valinor that the Noldor had left. Turgon and his people built Gondolin in secret. After it was completed, he took with him to dwell in the hidden city his entire people in Nevrast — almost a third of the Noldor of Fingolfin's House — as well as nearly three quarters of the northern Sindar.
The End of the City
The city stood for nearly 400 years until it was betrayed to Morgoth by Maeglin, Turgon's nephew. Maeglin was captured while mining outside the Encircling Mountains (against Turgon's orders). Maeglin betrayed the location of Gondolin after being promised Lordship and Turgon's daughter Idril. Morgoth then sent an army over the Crissaegrim the northern most precipitous and dangerous portion of the Encircling Mountains during The Gates of Summer (A great Gondolin festival) catching them unawares and sacking the city with relative ease. In addition to orcs, Balrogs and dragons, Melkor's (Morgoth's) army, in early versions of the story, included iron machines powered by "internal fires" and used as personnel carriers, to surmount difficult geographic obstacles and to defeat fortifications. The imagery of the Fall of Gondolin bears some similarity to the siege of Minas Tirith as told in The Lord of the Rings.