Durthang.

Durthang fortress was high up in the mountains, near Isenmouthe at the great spur of Ephel Dúath forming the south-western wall of Udûn in Mordor. The fortress was build after the War of the Last Alliance S.A.3429-3441 like the Tower of Cirith Ungol and the Teeth-Towers to watch and guard the passes and borders of Mordor against evil. While the twenty seventh King, Tarondor ruled in Gondor T.A.1636-1798, the watch on the borders of Mordor was ceased because of the Dark Plague and the fortresses that guarded the passes became unmanned and when the Ringwraiths took possession of Mordor, Durthang became a stronghold of Orcs.