The two great towers, Carchost to the east and Narchost to the west were built to guard the entrance into the Black Land. However, the vigilance of men failed and darkness crept back into Mordor. When the Dark Lord fled from Dol Guldur and openly declared himself once again in the Dark Tower, he reenforced and rebuilt the towers and created The Black Gate across the span between them. No one entered the Land of Shadow with out feeling the bite of Teeth of Mordor.
The Black Gate of Mordor is often thought to have been a gate built by the Númenóreans, to shield the forces of Mordor from exiting. It was, infact built by Sauron, to prevent invasion at the gap between the Ered Lithui and the Ephel Dúath. The Towers of the Teeth, Carchost and Narchost, were however built by the Men of Gondor as a guard after Sauron’s first defeat. The reconstruction of Minas Ithil, Tower of the Rising Moon, as well as the construction of Cirith Ungol were also done for the same purpose.
However, during the aftermath of the Kin-strife in Gondor the watchfulness of the guards in these strongholds relaxed. Thus the Ringwraiths and Orcs re-entered Mordor, eventually overrunning the garrisons and inhabiting them for themselves. It was at this time that the tower of Minas Ithil was taken by the Nazgûl, having its name changed to Minas Morgul, Tower of Sorcery.
During the War of the Ring, the Army of the West, numbering some 6,000 men, arrived at the Black Gate with the intention of drawing the Eye of Sauron away from Mount Doom, to allow Frodo the Ringbearer to cast the One Ring into the Crack of Doom within it. This they achieved, and the Ring was destroyed in the fires of Mount Doom, following which the Black Gate and the Towers of Teeth immediately collapsed.
