The Black Gate that closes off the main entrance to Mordor at the mouth of Cirith Gorgor, the "Haunted Pass".

The gate was first built shortly after Sauron first began fortifying Mordor after the year 1000 of the Second Age. The Morannon consisted of a huge rampart of stone into which was placed a single gate of iron with arched doors built into it.

In the Year 3434 of the Second Age the forces of the Last Alliance of Elves and Men defeated the forces of Sauron on the Dagorlad and entered Mordor through the Morannon.

After a seven year siege of Barad-dûr, Sauron was defeated and after that, from Gondor, a watch was maintained on Mordor. Towers were built on the sheer hills outside of the gate so that the forces of Gondor could maintain a constant watch on Mordor in the Third Age. But the Great Plague of TA 1636 so weakened and reduced the population of Gondor that the Towers were abandoned by TA 1640.

In TA 2951 Sauron openly declared himself in Mordor and began rebuilding his fortifications. The towers abandoned by Gondor were rebuilt and occupied by Sauron's minions and became known as " Carchost and Narchost ( Fang-fort and Fire-Tooth); the "Towers of the Teeth" of Morannon. The hills on either side of the Morannon were riddled with holes and caves and filled with Orcs and other fell creatures of Mordor. Beyond the towers, the Dagorlad was dotted with slag heaps and mires.

It was upon two of these slag heaps that Aragorn arrayed his forces to battle with the forces of Mordor in the Battle of the Black Gate on March 25 of TA 3019. It was on that date that the One Ring was returned to the Crack of Doom and when that happened the Towers of the Teeth crumbled , the Morannon was hurled into ruin, and all of the evil works held together by the power of Sauron were unmade.
Encyclopedia entry originally written by PotbellyHairyfoot