Aglarond and Angrenost (later Isengard) were the two fortresses built by Gondor guarding the Fords of Isen. Like Angrenost to the north it was initially well guarded, but as the population of Calenardhon (the later Rohan) dwindled it was not maintained and, losing importance, 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 Rohirrim care, who named it Súthburg (southern burg in their language). The Gondorian guard was merged with that of Angrenost to the north, which remained in the keep of Gondor. Guard duty of the Fords was initially shared between Gondor and Rohan, but later maintained only by the Rohirrim.
During the reign of Helm Hammerhand, Rohan was attacked by Dunlendings from across the river Isen. Edoras was taken, and Helm and many of his people retreated to Súthburg. Because Helm always blew a great horn before riding out to fight the beleaguering Dunlendings, the keep was renamed Hornburg. During the War of the Ring, the Hornburg was the refuge of the Rohirrim as they defended Helm's Deep.
After the war, Gimli the Dwarf, who fought in that battle, established a colony of Durin's folk there. Aglarond lay in front of a huge and very beautiful cave system, which proved irresistible to him. Gimli become known as 'the Lord of the Glittering Caves', although this new realm was presumably under the overlordship of Thorin Stonehelm, his kinsman and heir of the lordship of all Durin's Folk. The Dwarves of Aglarond restored the Hornburg following the War of the Ring, and it became a fortress they shared with the Rohirrim.
