Durthang was an old castle in northern Mordor. It stood high up in the northern Ephel Dúath above the valley of Udûn a few miles north of the Isenmouthe where a mountain spur branched away south-east to the Isenmouthe. A road ran winding down from Durthang and then turned east along a shelf in the southern side of the mountain spur down into the plain of Gorgoroth and along the feet of the mountain spur on to the Isenmouthe.
It is possible that Durthang was originally a fortress of Gondor. The Gondorians had constructed several fortresses overlooking the entrance to Mordor in the aftermath of the War of the Last Alliance like the Towers of the Teeth or the Tower of Cirith Ungol. After the Great Plague devastated Gondor in T.A. 1636, the watch on the borders of Mordor ceased and the fortresses that guarded the passes were unmanned in T.A. 1640 because of a lack of troops, which possibly included Durthang. It is possible that Durthang was later reoccupied and manned by troops from Gondor, because in T.A. 1944 when king Ondoher of Gondor and both his sons fell in a battle against the Wainriders north of the Morannon the Morannon and the Towers of the Teeth were still manned by troops from Gondor. It is possible that Durthang was occupied by Orcs in T.A. 1980 when the Witch-king came to Mordor and gathered the Nazgûl there. At the time of the War of the Ring Durthang was an Orcish stronghold.
War of the Ring
On 18 March, T.A. 3019, a troop of Orcs from Durthang caught up with a band of Orcs that was marching eastward in Mordor. Frodo Baggins and Samwise Gamgee were in that band, disguised as Orcs and they were forced to march along with the troop for some distance before they were able to slip away.