GondolinGondolin - After the first failed attempt to recover the Silmarils, the Noldor settled into Beleriand.  This led to the reunion of the Noldor with the Sindar.


"...and the Noldor met them with gladness, as kinsfolk long sundered; but speech at first was not easy between them, for in their long severance the tongues of the Calaquendi in Valinor and of the Moriquendi in Beleriand had drawn far apart.  From the Elves of Mithrim the Noldor learned of the power of Elu Thingol." ~The Silmarillion, chap. 13 (Of the Return of the Noldor)


However Thingol was not overly enthusiastic about the reunion.


"Now King Thingol welcomed not with a full heart the coming of so many princes in might out of the West, eager for new realms; and he would not open his kingdom, nor remove its girdle of enchantment, for wise with the wisdom of Melian he trusted not that the restraint of Morgoth would endure.  Alone of the princes of the Noldor those of Finarfin's house were suffered to pass within the confines of Doriath; for they could claim close kinship with King Thingol himself, since their mother was Eärwen of Aqualondë, Olwë's daughter."  ~The Silmarillion, chap. 13 (Of the Return of the Noldor)


And so the majority of the Noldor had to make homes for themselves; and two great cities were built, one modeled after Tirion, and the other after Menegroth (see Nargothrond below).  The one modeled after their home in Aman was named Gondolin.  It was built by Turgon, son of Fingolfin.


"But Turgon dwelt still for the most part in Nevrest, 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." ~The Silmarillion, chap. 15 (Of the Noldor in Beleriand)


And Gondolin was surrounded by a circle of mountains making it highly defensible.