The hidden city built by Turgon, Gondolin was the longest lasting of the Elven realms in Beleriand. It was built in the hidden valley of Tumladen within the Echorath. There was only one easy entrance, known as the "Way of Escape". The city was designed in memory of the city of Túna, and after time, equaled it.
The reason that Gondolin survived so long in the wars against Morgoth was that for many years nobody knew where it was. Only four people came into the city later than those who founded it. Those four were Maeglin, Eöl, Húrin and Huor. Also, Ulmo had long blessed the waters of the Sirion River.
Turgon took ten thousand of his forces to the Nirnaeth Arnoediad, revealing the existence of the city for the first time. However, the secret of the city's location remained. After the battle, Tuor was brought to Gondolin. Later Earendil was born to Idril.
The city was eventually betrayed by Maeglin, who had been captured
by Morgoth. The forces sent against Gondolin included: wolves,
orcs, dragons and Balrogs. The destruction was utter and complete.