The Towers of the Teeth (or Teeth of Mordor) are two towers situated on either side of the Morannon. The two towers, called Carchost and Narchost, that stand on either side of Cirith Gorgor, the Haunted Pass, which lies between the Ephel Dúath (Mountains of Shadow), and the Ered Lithui (Ash Mountains).
Originally built by Men of Gondor following the downfall of Sauron at the end of the Second Age, the Towers of the Teeth were eventually taken over by the servants of the Dark Lord. They were then repaired and fortified and were incorporated into the defenses of the pass and gate.
Tolkien's writings leave it unclear which tower was located on which side of the Morannon. However, Carchost appears to have stood to the eastern side and Narchost on the western side. Frodo, Sam, and Gollum beheld these towers and the Morannon, the Black Gate, between them as they debated the best way to enter Mordor, and it was below these towers that the Captains of the West and their small army made their last stand against the forces of Sauron. The towers were presumably destroyed in the War of the Ring with the defeat of Sauron following the destruction of the One Ring. Their collapse is shown in Peter Jackson's interpretation of Return of the King.
There are no definitive records to this effect, but some say that Carchost, which means "fang fort" in Sindarin, stood on the east (or north) side of the Morannon and Narchost, which means "fire fort" in Sindarin, stood on the west (or south).