§ Of Tuor and the Fall of Gondolin The Silmarillion

Thus it was that the son of Huor rode across Tumladen, and came to the gate of Gondolin; and passing up the wide stairways of the city he was brought at last to the Tower of the King, and looked upon the images of the Trees of Valinor. 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.