Of Gondolin's Ending

Some time after the building of Gondolin Meglin, Turgon's Sister-son came to Gondolin and was welcomed by Turgon. Also Ulmo purposed that Tuor, Huor's (bother of Húrin) son, should come to Gondolin bearing a message to the King. Again with no extended history Ulmo guides Tuor's path until he meets a wandering Elf of Gondolin who leads him to the secret city (no fancy armor left by Turgon to mark the coming of the prophet).

The message given by Ulmo is very different in the Sketch and Quenta than what it is in the Silmarillion. Essentially Turgon is give two choices:

• Lead his armies in open war against Morgoth. To which Ulmo promised to succor the help of the Valar and even though it was to be a terrible strife Turgon would break Morgoth's forces. In one account he states the Orcs would perish to never again harass Elves and Men.
• The other option was to vacate Gondolin and flee West to Valinor. Again Ulmo promised to aid Turgon's people across the sea and win their pardon from the Valar

Both very sound options. But Turgon in his pride and with the unwise counsel of Meglin chooses to trust in the secrecy and strength of Gondolin and do nothing. He does not know that Meglin secretly desires his throne and his daughter. 

However Idril's heart was turned toward Tuor and the two were married. She filled with unrest ordered a secret tunnel built. Meglin, now embittered, wandered from Gondlin and was captured by Morgoth. The Sketch is far more sparse in details but it's implied that Meglin gives up Gondolin's secrets because of his lust for Idril and Morgoth's promise to give him her and Gondolin after it's sack. In the Quenta Meglin finally breaks after much torture but is promised the same reward.

The sack of Gondolin follows with Tuor, Idril, their son Eärendel escaping along with other refugee's. In both accounts many people of Gondolin flee through the Way of Escape only to be slaughtered by a dragon Morgoth had wait there being told of this secret escape route by Meglin. In the Silmarillion Turgon had this route closed off after refusing Ulmo's message. The image, much more tragic, of people thinking their are escaping only to find death at the claws, teeth, and fire of a dragon is much more compelling to me.