One of the towers known as the Teeth of Mordor, that stood on two hills either side of the Black Gate of the Morannon, guarding the northwestern entrance into Mordor. Narchost and its companion Carchost were originally built by the Gondorians, as a guard on Mordor after Sauron's first defeat. As Gondor's strength waned, Narchost was abandoned and left to decay, until Sauron returned to his ancient land. His forces took over both the old guard-towers and repaired them, so that at the time of the War of the Ring, they formed part of Sauron's own defences.
The Lord of the Rings never states whether Narchost was the western or eastern of the Towers, but a variety of circumstantial evidence points to it being the tower that stood to the west of the Morannon.