Gondolin

Warning: Wikipedia contains spoilers

Ulmo the Lord of Waters revealed the location of the Vale of Tumladen to 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 Dry River that eventually 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, taking 52 years. It 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.

Gondolin fell in F.A. 511, betrayed to Morgoth by Maeglin, and only a few of the inhabitants escaped through Idril's secret passage.