Then Tuor looked down upon the fair vale of Tumladen, set as a green jewel amid the encircling hills; and he saw far off upon the rocky height of Amon Gwareth Gondolin the great, city of seven names, whose fame and glory is mightiest in song of all dwellings of the Elves in the Hither Lands.
Then Tuor stood before Turgon son of Fingolfin, High King of the Noldor, and upon the King's right hand there stood Maeglin his sister-son, but upon his left hand sat Idril Celebrindal his daughter; and all that heard the voice of Tuor marvelled, doubting that this were in truth a Man of mortal race, for his words were the words of the Lord of Waters that came to him in that hour. And he gave warning to Turgon that the Curse of Mandos now hastened to its fulfilment, when all the works of the Noldor should perish; and he bade him depart, and abandon the fair and mighty city that he had built, and go down Sirion to the sea. But Turgon was become proud, and Gondolin as beautiful as a memory of Elven Tirion, and he trusted still in its secret and impregnable strength, though even a Vala should gainsay it (...) Maeglin spoke ever against Tuor in the councils of the King, and his words seemed the more weighty in that they went with Turgon's heart; and at the last he rejected the bidding of Ulmo and refused his counsel.
And Tuor remained in Gondolin, for its bliss and its beauty and the wisdom of its people held him enthralled; and he became mighty in stature and in mind, and learned deeply of the lore of the exiled Elves. Then the heart of Idril was turned to him, and his to her; and Maeglin's secret hatred grew ever greater, for he desired above all things to possess her, the only heir of the King of Gondolin.
The Silmarillion (chapter 23)