In the northwest, the pass of Cirith Gorgor led into the enclosed plain of Udûn. Sauron built the Black Gate of Mordor (the Morannon) across the pass. This added to the earlier fortifications, the Towers of the Teeth - Carchost to the east, Narchost to the west, guard towers which had been built by Gondor to keep a watch on this entrance. The passage through the inner side of Udûn into the interior of Mordor was guarded by another gate, the Isenmouthe. Outside the Morannon lay the Dagorlad or Battle Plain, and the Dead Marshes.