The last of the Seven Gates of Gondolin, set at the end of the passage of the Orfalch Echor as it ran down out of the Encircling Mountains onto the concealed plain of Tumladen. It lay just a short way along the road from the Sixth Gate, the Golden Gate, and was the final step on a journey through the mountains: it was said that no traveller who passed the Seventh Gate would ever be permitted to return.
The Seventh Gate was also known as the Great Gate, the Last Gate or, as it was formed from a fence of steel spikes, the Gate of Steel. The Gate's metal fence stretched from one side of the valley to the other, and at each end, where it met the valley wall, rose a seven-storey tower of steel. Across the span of the fence stood seven steel spikes rising from the ground, and the fence itself was formed from seven horizontal bars connecting these spikes, and reinforced by forty-nine vertical rods of steel, with seven of these rods between each of the larger spikes. In the centre of the Seventh Gate, above its central spike, rose an image of the royal helm of Turgon, King of Gondolin, surrounded by diamonds.
The silver-armoured guardians of the Seventh Gate occupied the turrets that stood on either side, each bearing a helm with a tall steel spike emerging from its crest. These guardians served under a Warden, a role that was held for some time by the famous warrior known as Ecthelion of the Fountain. The Gate could not be opened without the Warden's permission: at his command, the bars of the central section would swing inward, opening the way across the plain to Gondolin on its hill. At the same time, trumpets would be sounded from the towers to warn the city that a visitor was entering its secret territory.
According to the unfinished Of Tuor and his Coming to Gondolin (Unfinished Tales Part One, I), the Seventh Gate was wrought by Maeglin after the Nirnaeth Arnoediad. This might be taken to imply that there were just six gates before that point in history. However, in the Silmarillion's account of Maeglin's arrival in Gondolin, long before the Nirnaeth, it's specifically said that he and his mother Aredhel passed through Seven Gates. The implication seems to be that Gondolin always had Seven Gates, but after the Nirnaeth the original Seventh Gate was replaced by the Maeglin'sGate of Steel.