Mordor is bordered by a semi-circle of mountains, the Ered Lithui (Ash Mountains) to the North and the Ephel Dúath (Shadowy Mountains) to the West and South. The Morannon closes the gap between them, a high stone rampart running across Cirith Gorgor with a gigantic iron gate set into it. It is guarded at one end by Narchost and at the other by Carchost, known together as the Towers of the Teeth, one of which can be seen in the background of the picture. The gate was first built by the men of Gondor to keep watch on Mordor at the beginning of the Third Age.
It is to this gate that Gollum first leads Frodo and Sam on their way to the volcano Orodruin in Mordor. As can be seen in the painting, the wall is paced by sentinals, and is built on hills riddled with Orc caves. At one command large armies could issue forth from these caves and from the gate itself. Frodo and Sam realised that the path was impassable, so allowed Gollum to lead them into Mordor through the spider Shelob's lair.
It was at the gate of the Morannon that Aragorn gathered his army and Gandalf spoke with the Mouth of Sauron. The wall and gate were ruined when the Ruling Ring was destroyed and its works undone.