It was originally built by Gondor after the war of the Last Alliance of Elves and Men to keep watch on Mordor and prevent anyone or anything from entering there again. At first Durthang was like the other fortresses and castles built to watch over Mordor, fully maintained and manned but as Gondor began to lose its power and influence beginning in the 15th century, it slowly fell into decline. It was finally abandoned in TA 1640 after the Great Plague that vastly depopulated Gondor. However Gondor recovered but Sauron later used the Wainriders to distract Gondor, Durthang was soon corrupted by Sauron upon his return to Mordor, used by the Nazgûl, and finally used by the orcs and the Haradrim upon the beginning of the War of the Ring.
Frodo and Sam passed this castle when they headed to Mount Doom as they had to go northwards and then south to avoid Barad-dûr, and they found this castle to be vigilant and hostile. The castle sent out orc-forces to march to Udûn during the attack on Mordor during the War, and also supplied forces during the Battle of the Morannon, the battle which ended the War of the Ring in the south-east. The fate of Durthang remains unknown. It was possibly destroyed in the climax of the war, yet because it was built by the Men of Gondor it possibly survived and may have later been used by the freed slaves of Mordor as a base.
Durthang was a Sindarin word that meant 'Dark-Oppression'.