The round valley of Tumladen, within the Encircling Mountains, had originally been a lake, and in its centre stood a hill that had once been an island: Amon Gwareth. In the fiftieth year of the First Age, Turgon journeyed from his halls in Nevrast with his cousin Finrod, and was guided by Ulmo to the hidden valley of Tumladen and there he founded Gondolin, as a memorial to ancient Tirion that lay beyond the Great Sea.
The first building of the city took fifty-two years to complete. After this time, the people of Turgon, a great number of Noldor and Sindar, traveled from Nevrast, where they had dwelt, and secretly entered the valley of Tumladen. Turgon gave his city the name Ondolindë (Quenya for "The Rock of the Music of Water", because of the fountains of Amon Gwareth), but in Sindarin this was rendered Gondolin, the Hidden Rock.
After their arrival in the new city, the Gondolindrim continued to labour in its building, until it was said to rival even Tirion itself. Its walls stood high and white above the plain, and its most prominent feature was the great Tower of the King, where, among the fountains, Turgon himself made Glingal and Belthil, trees of gold and silver made in memory of the Two Trees of Valinor.
There followed two centuries of happy peace: Morgoth was besieged in the far north of the world, and the people of Gondolin lived undisturbed by the events outside their fair city. At last, though, a seed of discontent appeared: King Turgon's sister Aredhel determined to leave the city, much against Turgon's wishes, and journey into Middle-earth. Soon after her departure, her guards returned, and reported that they had lost her in the dark and sorcerous region to the south east of the city, known as Nan Dungortheb.
More than twenty years then passed, and then suddenly Aredhel returned. With her was one who she claimed to be Maeglin, her son by Eöl the Dark Elf of Nan Elmoth. This Maeglin accepted Turgon as lord, but his father Eöl had followed his wife and son to Gondolin and been captured at the entranceway. He was brought before Turgon. Eöl refused to submit to Turgon's authority, and instead chose death for himself and his unwilling son. He threw a poisoned dart to slay Maeglin, but instead struck Aredhel, who fell ill with the poison and died. The body of Eöl was broken on the Caragdûr for this.
Maeglin, though, had had no part in these evils, and Turgon accepted him, and he grew to be among the great in Gondolin, wise in council, cunning in smithcraft and mighty in battle.
Eöl and Aredhel meeting in the Forest of Nan Elmoth, by Noldomirwen
For more than a hundred years after the deaths of Aredhel and Eöl, Gondolin again had peace. The time was coming, though, when Morgoth would break the Siege of Angband, and the unstoppable doom of the Elves would fall upon them. One winter's night, the fires of Angband destroyed the leaguer of the Noldor: the Dagor Bragollach. In this disaster, the people of Gondolin played no immediate part.
They were drawn into the events of those years, though, when two young brothers of the race of Men, Húrin and Huor, were cut off from their army and became lost amid the feet of the Crissaegrim. Thorondor brought them to Turgon. At the bidding of Ulmo, Turgon accepted them, and they remained in Gondolin for almost a year, when they returned to their homes. In this kindly act were the first seeds of Gondolin's destruction, more than fifty years later.
Huor & Húrin flying to Gondolin, by Alan Lee.
Turgon now devised a new policy for the salvation of the Elves: he began secretly to send his people out westward across the great sea, to seek the land of the Valar and ask their pardon and aid. None of his mariners succeeded, but this was a wise course, though Turgon never knew it: he was lost long before his grandson Eärendil, aided by a Silmaril, finally succeeded in this task.
As time passed since the destruction of the Dagor Bragollach, the Elves of Beleriand began to arm for a counterstroke, and Turgon secretly began his own preparations. Fourteen years after the breaking of the Siege of Angband, and some three hundred and fifty since the completion of Gondolin, Turgon rode for the first time to war. Unknown and unbidden by his kin, he rode to their aid with an army of ten thousand. This was to be the great battle that was to become known as the Nirnaeth Arnoediad.
'The Hill of Slain', by Ted Nasmith - after Nirnaeth Arnoediad.
Morgoth poured much of his strength and resources into discovering the location of the Hidden City, and his spies infested lands about the Encircling Mountains and wherever he could penetrate. After Nargothrond fell, Gondolin was the last great stronghold of the Noldor. The unbidden coming of Maeglin precipitated the destruction of the great city.
The first great blow to the security of Gondolin came by accident. Húrin, who was once held captive by Morgoth, was released to wander in the world, unconsciously furthering the Dark Lord's cause. He came to the edge of the Mountains, hoping that an Eagle would bear him to Gondolin. But Turgon, afraid for the lives of those in his city, and rightfully fearing what Morgoth might have done to Húrin, withheld rescue for too long. Húrin, seeing nothing, cried out in a loud voice "Turgon, Turgon, remember the Fen of Serech! O Turgon, will you not hear in your hidden halls?". Morgoth now knew the general area in which Gondolin lay, for his spies were watching this. Húrin turned away, broken and bitter.
Ulmo, the Patron of the city, watched with sad eyes, foreseeing the doom of Gondolin. He called forth Tuor, son of Huor, to the Sea, where he gave him a message for Turgon. Tuor delivered this message faithfully, by the aid of Ulmo and an elf named Voronwë, who guided him to the city. Ulmo's advice was that Turgon must abandon the city and seek the sea. For though hitherto the Valar had barred Turgon's messengers from reaching the Undying Lands, Ulmo foresaw that a direct descendant of the lords of the Noldor might be allowed to pass to the lands of the West. Turgon, because of his pride and the love of his city, decided to ignore this warning. Tuor, however, was welcomed in the city by all save Maeglin. Unwittingly, Turgon, who had always been so careful about strangers, further advanced the events that would lead to the fall of of Gondolin by allowing Tuor to stay. For Tuor and Idril, the King's daughter, fell in love.
It was an extremely rare thing for a man to wed an elf-woman, but Turgon, who loved Tuor as a son, permitted it when he found that his daughter was full willing. Maeglin hated Tuor for this, and plotted his revenge on Idril.
While searching for ore and straying (against the commands of Turgon) too far from the city, Maeglin was captured by Orcs. At his begging and bribing they took him before Morgoth himself, who extracted the information about Gondolin at a price: the death of Tuor and Eärendil, the hand of Idril, and the lordship of Gondolin after its capture.
Maeglin returned to Gondolin and kept his capture by Morgoth a secret. Idril, however, who had noticed and rejected his advances, saw and feared the change that had come over him. Therefore she discussed with her husband the idea of making an escape tunnel. Tuor, confused but willing, complied. In secret, a tunnel that cut through the rock of the hill and out under Tumladen was constructed.
At last Morgoth finalized his plan, and he loosed his massive army upon the city. Again Tuor and others urged Turgon to flee the brunt of the assault, but, convinced of the invincibility of his city (and with Maeglin whispering in his ear), Turgon decided to stay and fight.
There was a great siege, during which Maeglin sought (from the inside) to take Idril and throw Eärendil from the walls. But Tuor arrived in time to save them both, and after a brief struggle hurled Maeglin down into the flames. But the city could not be saved, and thousands were killed. One by one the great lords of Gondolin fell, and Turgon, disillusioned and broken-hearted, ordered all to follow Tuor out through the tunnel when he saw that the destruction was inevitable. Only his guard stayed around him, and all who remained were killed as the white tower collapsed.
Gathering as many of the people as they could find, Tuor and Idril escaped down the tunnel. It was a hard road through the mountains; Glorfindel was killed by a balrog that lay in ambush. But at last the Gondolindrim came to Nan-tathren, and after resting there for some time they came down to the refuge at the Mouths of Sirion, and mourned the loss of the White City.
'Fall of Gondolin', by Ted Nasmith