Gondolin - One of the two great cities of the Noldor that was established in Beleriand which stood for almost five centuries before it was sacked by the forces of Morgoth.  It was built by Turgon, son of Fingolfin.  The key to Gondolin's safety lie in its secrecy from the outside world.  It was surrounded by a circle of mountains and accessible by only a single pass carved out by a river.  But its location was finally betrayed by Maeglin, and Morgoth sent an army to destroy it.

"But behind the circle of the mountains the people of Turgon grew and throve, and they put forth their skill in labour unceasing, so that Gondolin upon Amon Gwareth fair indeed and fit to compare even with Elven Tirion beyond the sea.  High and white were its walls, and smooth its stairs, and tall and strong was the Tower of the King.  There shining fountains played, and in the courts of Turgon stood images of the Trees of old, which Turgon himself wrought with Elven-craft..." ~The Silmarillion, chap. 15 (Of the Noldor in Beleriand)