The Black Gate was set in an impregnable black stone and iron wall that stretched from the Mountains of Ash in the north to the Ephel Duath (Mountains of Shadow) in the west. The wall has been estimated to be 60 feet high and 250 feet long with each half of the great gate being 90 feet wide and set on large stone wheels. Behind the gate were gigantic circular stone ramparts, and when the gate needed to be opened, two pairs of Mountain-trolls who were tethered to gigantic beams pushed their way around their rampart's track, gradually levering open the gate and allowing for the incoming or outgoing of Mordor’s armies.
Inset within the wall were myriads of archers, spearmen, bowmen, ballistae, and hundreds of thousands of Orc troops ready to defend Mordor. The Gates themselves may have been made of the same indomitable stone that constitutes Orthanc, so hard that even the Ents could not dent it.
It was originally a gate built by Sauron the Dark Lord of Mordor in the Second Age, to protect and guard the northern entrance into Mordor and to prevent invasion at the gap between the Ered Lithui (Ash Mountains) and the Ephel Duath (Mountains of Shadow). It was probably built with the help of the power of the One Ring, like the Barad-dûr. After Sauron's fall after the Battle of Dagorlad, it became a garrison of the Men of Gondor. In Gondor's early days, when it was building towers and cities such as Minas Ithil and Cirith Ungol close to Mordor's border, it raised the two great Towers of the Teeth, Narchost and Carchost, which were built on either side of the wall and were tall enough to overlook it.
It was backed up on the other side by the Isenmouthe, and protected by the castle of Durthang to the west; it was redesigned to prevent anything evil from entering or leaving Mordor, shielding the outside from it - and it from the outside. The reconstruction of Minas Ithil, Tower of the Rising Moon, as well as the construction of Cirith Ungol was also done for the same purpose.
However, they eventually failed and they were taken over by the servants of Sauron. Once taken by the Enemy, they became a monument of dread, fear, evil, and awe by anyone who saw the gate. Thus the Ringwraiths and orcs re-entered Mordor, eventually overrunning the garrisons and inhabiting and taking them for their own uses. It was at this time that the Nazgûl took Tower of Sorcery the tower of Minas Ithil having its name changed to Minas Morgul.
During the War of the Ring, Frodo and Sam had a chance to go through the Black Gate when they arrived at Mordor, but it was so heavily fortified and guarded that they turned away and were led on a different route into Mordor by Sméagol. They also witnessed an army of Easterlings enter the gate.
The Black Gate was the site of the last valiant battle of the War of the Ring, in the east. In March 3019, the Army of the West commanded by Aragorn Elessar and numbering some 6,000 men (plus one elf, one dwarf, and an Istar) and a hobbit arrived at the Black Gate and challenged the many remaining armies of Mordor. Their intention was to draw the Eye of Sauron away from Mount Doom, to allow Frodo the Ring-bearer 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 the Teeth immediately collapsed.
- The Atlas of Middle-earth pg. 92