§ Of the Noldor in Beleriand The Silmarillion

Now after the Dagor Aglareb the unquiet that Ulmo set in his heart returned to him, and he summoned many of the hardiest and most skilled of his people, and led them secretly to the hidden vale, and there they began the building of the city that Turgon had devised; and they set a watch all about it, that none might come upon their work from without, and the power of Ulmo that ran in Sirion protected them. But Turgon dwelt still for the most part in Nevrast, until it came to pass that at last the city was full-wrought, after two and fifty years of secret toil. It is said that Turgon appointed its name to be Ondolindë in the speech of the Elves of Valinor, the Rock of the Music of Water, for there were fountains upon the hill; but in the Sindarin tongue the name was changed, and it became Gondolin, the Hidden Rock. Then Turgon prepared to depart from Nevrast and leave his halls in Vinyamar beside the sea; and there Ulmo came to him once again, and spoke with him. And he said: 'Now thou shalt go at last to Gondolin, Turgon; and I will maintain my power in the Vale of Sirion, and in all the waters therein, so that none shall mark thy going, nor shall any find there the hidden entrance against thy will. Longest of all the realms of the Eldalië shall Gondolin stand against Melkor. But love not too well the work of thy hands and the devices of thy heart; and remember that the true hope of the Noldor lieth in the West and cometh from the Sea.'