Fall of GondolinFall of Gondolin (510 Y.S.) - The Noldor city of Gondolin was protected by the Encircling Mountains, also called Echoriath, and accessible by only a single hidden passage that had been carved out by a river.  The Noldor built seven gates across the passage in order to protect their city.  But Gondolin's greatest defense was its secret location, and King Turgon guarded that secret steadfastly (as was evidenced by his refusal to allow Eöl the Dark Elf to leave once he found the city).


However that secret was finally betrayed by Eöl's son, Maeglin, after he was captured by Orcs while mining for metal in the Encircling Mountains.


"Now on a time, when Eärendil was yet young, Maeglin was lost.  For he, as has been told, loved mining and quarrying after metals above all other craft; and he was master and leader of the Elves who worked in the mountains distant from the city, seeking after metals for their smithying of things both of peace and war.  But often Maeglin went with few of his folk beyond the leaguer of the hills, and the King knew not that his bidding was defied; and thus it came to pass, as fate willed, that Maeglin was taken prisoner by Orcs, and brought to Angband.  Maeglin was no weakling or craven, but the torment wherewith he was threatened cowed his spirit, and he purchased his life and freedom by revealing to Morgoth the very place of Gondolin and the ways whereby it might be found and assailed." ~The Silmarillion, chap. 23 (Of Tuor and the Fall of Gondolin)


And so, while Morgoth's assault on Beleriand was waging, he gathered his army and unleashed it upon the city he sought so long to destroy.


Fall of Gondolin"At last, in the year when Eärendil was seven years old, Morgoth was ready, and he loosed upon Gondolin his Balrogs, and his Orcs, and his wolves; and with them came dragons of the brood of Glaurung, and they were become now many and terrible.  The host of Morgoth came over the northern hills where the height was greatest and the watch least vigilant, and it came at night upon a time of festival...and there was no stay in the advance of the foe until they were beneath the very walls of Gondolin, and the city was beleaguered without hope.  Of the deeds of desperate valour there done, by the chieftains of the noble houses and their warriors, and not least by Tuor, much is told in The Fall of Gondolin: of the battle of Echtelion of the Fountain with Gothmog Lord of Balrogs in the very square of the King, where each slew the other, and of the defence of the Tower of Turgon by the people of his household, until the tower was overthrown; and mighty was its fall and the fall of Turgon in its ruin." ~The Silmarillion, chap. 23 (Of Tuor and the Fall of Gondolin)


King Turgon engaged Gothmog (who slew Fëanor) and both were killed.  Now Morgoth's destruction of Beleriand and the Noldor was almost complete...almost.