When Cirion, Steward of Gondor, gave Calenardhon to the Éothéod, Aglarond was transferred into the care of the Rohirrim, who named it Súthburg (southern burg in their language).
After Calenardhon was given to the Éothéod by Cirion, Steward of Gondor, and became Rohan, Isengard was the sole fortress retained by Gondor north of the Ered Nimrais (excluding Anórien), although Gondor almost forgot about it.