The borders of Beleriand were protected from the North by a line of mountains running from the Great Sea in the west as far as the March of Maedhros and Maglor's Gap in the east. There were few safe passes through these mountains, but the largest was a deep valley formed by the river Sirion as it flowed down from its northern springs. Between Ered Wethrin to the west and the Echoriath and Ered Gorgoroth to the east, this pass offered a means of passage through the mountains from the North onto the plains of West Beleriand.

This valley between the mountains was most commonly called the Pass of Sirion, but it was also known as the Narrow Land, a strip of river valley some thirty miles from end to end. It was watched by the tower of Minas Tirith, raised on the river island of Tol Sirion by Finrod Felagund to guard against the forces of Angband. After the Dagor Bragollach, the tower was captured by Sauron, opening the Narrow Land for the passage of the Orcs into Beleriand, and allowing them to ravage the lands southward of the Pass of Sirion.