Weta Workshop The Lord of the Rings Trilogy - The Black Gate Environment

Iron-wrought by thralls of Sauron™, the Black Gate, a thing of sharp and rigid design, spanned the narrow gap between the mountains that bound Mordor™. 

By this way alone might entry to Mordor™ be gained, lest those who sought to cross the threshold travel hundreds of miles east or south. The great gate was an impenetrable barrier, opened only by the command of its gatekeeper and the might of a team of Mountain Trolls, who alone were strong enough to move the blockade.