As recounted in The Silmarillion, the Vala Ulmo, the Lord of Waters, revealed the location of the Vale of Tumladen to the Noldorin Lord Turgon in a dream. The vale was in the Echoriath, the Encircling Mountains, just west of Dorthonion and east of the River Sirion. It was a round level area with sheer walls on all sides, with a ravine and tunnel leading out to the southwest called the Hidden Way, through which ran the empty riverbed of the Dry River, which once flowed into Sirion.

In the middle of the vale was the steep-sided hill of Amon Gwareth, upon which Turgon and his people built Gondolin. They took 52 years in building the city, and then the entire population of Nevrast, the old home of Turgon's people, secretly removed there.

Gondolin had a wall all around the crest of the hill, a main gate on the west, and a smaller gate to the north. Later Idril had a secret tunnel excavated, leading northwest to a difficult path through the mountains.

The Hidden Way was protected by seven gates constantly guarded; the first of wood, then stone, bronze, iron, silver, gold, and steel.

Another protection of Gondolin were the Eagles of Thorondor: these made their eyries in the Crissaegrim in the Encircling Mountains, and watched for spies.

Gondolin was divided in several quarters, all of which had their own leaders: these included Echtelion of the Fountain, and Glorfindel.

Gondolin was kept secret even from the other Noldor of Beleriand, and in all its history only a few strangers entered the city: these included Huor and Húrin, who were carried there by the Eagles, and Maeglin, sister-son of Turgon and Maeglin's father Eol. Much later Tuor entered the city, fulfilling an old prophesy by Ulmo.

Lúthien saw the city of Gondolin from high above, as she and a wounded Beren were flown from Angband to Doriath by an Eagle.