Carach Angren or Isenmouthe was a pass in the northwest of Mordor. The pass was formed where spurs reaching out from the ranges of the Ephel Dúath and the Ered Lithui met, leaving only a narrow passage between the Plateau of Gorgoroth and the smaller valley of Udûn to the north.
Description
As the passage from Gorgoroth to Udûn, Carach Angren was heavily fortified, and both the rocky spurs that overlooked it carried forts and towers. Across the passage itself, a wall of earth had been built, and a great ditch had been dug across the opening spanned by a single bridge. A great fence of pointed iron posts closed the gap leading into Udûn.