The Ephel Dúath, or the Mountains of Shadow, were a range of mountains that guarded Mordor's western and southern borders.
There were only two known passes through them: the "Nameless Pass" that ran between Minas Morgul and Mordor, and the nearby Pass of Cirith Ungol. The Morannon was the only pass through Mordor's outer mountain chains through which large armies could easily move, but apparently small amounts of troops were able to slowly funnel through the Nameless Pass. However, the Pass of Cirith Ungol was extremely inaccessible, involving climbing up or down the section called the Straight Stair, and then the Winding Stair, making it practically impossible for easy travel of large numbers of troops, much less horses and wagons.
On 10 March T.A. 3019, Frodo Baggins, Samwise Gamgee, and Gollum began their ascent of the stairs to Shelob's Lair. The hobbits, abandoned by Gollum, entered the lair on 12 March and Frodo was captured by Orcs the next day. On 14 March, Sam helped Frodo to escape from the Tower of Cirith Ungol and thus they completed their harrowing passage of the Ephel Dúath.