Pass of Aglon
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It was about six leagues in length, with the western side containing high walls and the eastern walls bordering the Dorthonion Plateau. Sons of Fëanor, Celegorm and Curufin, fortified this area with extra forces in Himlad, and about ten leagues to the east, the pair's elder brother Maedhros had upon the Hill of Himring his stronghold, thus protecting this weak point in the chain of kingdoms created by the Noldor after the Dagor-nuin-Giliath(Battle-under-Stars).
Regardless, the Battle of Sudden Flame took out Celegorm and Curufin's fortifications, leaving Maedhros' stronghold the center of a re-grouping of Fëanor's sons, and they later took back the Pass. Though after Nirnaeth Arnoediad Morgoth's domination consumed it, and it was never recovered, along with all of the land of northern Beleriand.