Teeth of Mordor

In the days after Sauron's first expulsion from Mordor, the Gondorians built two watch-towers to guard the pass of Cirith Gorgor against his return. These two towers, named Carchost and Narchost, stood on the cliffs above the pass. As the centuries passed, Gondor's watch on Mordor failed, and at last the Towers of the Teeth were left unmanned. When Sauron's forces returned to his ancient home, they found the watch-towers that were meant to guard against them empty, but they were quickly occupied. So, at the time of the War of the Ring, the Teeth of Mordor were part of Sauron's own defences.