§ Of Beleriand and Its Realms The Silmarillion

East of Dorthonion the marches of Beleriand were most open to attack, and only hills of no great height guarded the vale of Gelion from the north. In that region, upon the March of Maedhros and in the lands behind, dwelt the sons of Fëanor with many people; and their riders passed often over the vast northern plain, Lothlann the wide and empty, east of Ard-galen, lest Morgoth should attempt any sortie towards East Beleriand. The chief citadel of Maedhros was upon the Hill of Himring, the Ever-cold; and that was wide-shouldered, bare of trees, and flat upon its summit, surrounded by many lesser hills. Between Himring and Dorthonion there was a pass, exceeding steep upon the west, and that was the Pass of Aglon, and was a gate unto Doriath; and a bitter wind blew ever through it from the north. But Celegorm and Curufin fortified Aglon and held it with great strength, and all the land of Himlad southward between the River Aros that rose in Dorthonion and his tributary Celon that came from Himring.