Three sides of Mordor were bounded by mountain ranges, arranged in a rough rectangle: Ered Lithui, translated as 'Ash Mountains' in the north, the Ephel Dúath ("Fence of Shadow") in the west, and a range of unknown name in the south. In the northwest the pass of Cirith Gorgor led into the enclosed plain of Udûn. Sauron built the Black Gate of Mordor (the Morannon) across the pass, joining the Towers of the Teeth, two earlier guard towers built by Gondor to keep a watch on this entrance. The passage through the inner side of Udûn into the interior of Mordor was guarded by another gate, the Isenmouthe. Outside the Morannon lay the Dagorlad or Battle Plain.
In the interior within this mountainous border lay Sauron's main fortress Barad-dûr, the arid plateau of Gorgoroth, and Mount Doom. To the east lay the plain of Lithlad. A narrow pass led through the Ephel Dúath, guarded by Minas Morgul (earlier Minas Ithil). A higher, more difficult pass, Cirith Ungol, just to the north, was guarded by a tower originally built by Gondor. This pass, "the pass of the spider", was also blockaded by Torech Ungol, the lair of the giant spider Shelob. The fortress Durthang lay in the northern Ephel Dúath above Udûn.
Núrn, the southern part of Mordor, was less arid and more fertile. Streams here fed the salt Sea of Núrnen. This region was farmed by Sauron's slaves to support his armies.
To the west of Mordor was the narrow land of Ithilien, to the northeast Rhûn, and to the southeast, Khand. To the northwest lay the Dead Marshes.
Inside the Ephel Dúath ran a lower parallel ridge, the Morgai, separated from the Ephel Dúath by a narrow valley that Frodo and Sam followed northward after escaping from Cirith Ungol. Water trickled into this vale from the Ephel Dúath, and the texts describe it as a "dying land not yet dead". The vegetation included "low scrubby trees", "coarse grey grass-tussocks", "withered mosses", "great writhing, tangled brambles", and thickets of briars with long, stabbing thorns. The fauna included maggots, midges, and flies marked with "a red eye-shaped blotch".