The Towers of the Teeth (or Teeth of Mordor) are two towers situated on either side of the Black Gate. The two towers, called Carchost and Narchost, that stand on either side of Cirith Gorgor, the Haunted Pass, which lies between the Ephel Dúath, and the Ered Lithui.
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 Black Gate. 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 Black Gate, 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 destroyed in the War of the Ring with the defeat of Sauron following the destruction of the One Ring.
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 Black Gate and Narchost, which means "fire fort" in Sindarin, stood on the west (or south).
A song was written mentioning these towers, and showing the confidence of victory Sauron's army feels, even saying those who fight against them have only a choice of evils.