After the publication of The Lord of the Rings, Tolkien wrote about the history of Rohan, in writings now collected by his son Christopher in Unfinished Tales. These state that the fortresses of Aglarond and Angrenost (renamed Isengard by the Rohirrim) were built by Gondor to guard the shoulders of the Gap of Rohan. Like Angrenost to the north, it was initially well guarded, but as the population of Calenardhon dwindled it was not maintained and was left to a hereditary small guard who intermarried with Dunlendings. When Cirion, Steward of Gondor, gave Calenardhon to the Éothéod, Aglarond was transferred into the care of the Rohirrim, who named it Súthburg (South-fortress in Old English). The Gondorian guard was merged with that of Isengard. Guard duty of the Fords was initially shared between Gondor and Rohan, but later maintained only by the Rohirrim.