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The valley was described as being blocked over its entire width by the natural series of hills called Helm's Dike and behind that lay the fortress of Aglarond or the Hornburg, at the entrance to the Glittering Caves.
In the Appendices of The Lord of the Rings, the history of the valley states that it was home to a small Gondorian regiment, but after the place which was called Calenardhon became Rohan, it became an encampment of the Rohirrim, where the guards of the Fords of Isen lived.
It also states that during the war with the Dunlendings under Wulf, the Rohirrim under King Helm Hammerhand and many of his people sought refuge in the keep, where they held out during the winter of T.A. 2758–2759.
The castle itself was described as having a long causeway which wound up to the great gate of the fortress itself. Inside the keep there were stables and an armoury, as well as a great hall in the rear which was dug out of the mountainside. There also was a great tower (the Hornburg) the top of which consisted of the great horn of Helm Hammerhand. The Deep, which was described as standing next to the fortress, was barred by the long Deeping Wall, which was described as consisting of solid rock except for a small culvert which allowed water from the Deeping Stream to enter; this rendered a fresh supply in sieges of great length. The Deeping Wall itself is described as standing 20 feet tall; and also being wide enough for four men to stand side by side. Access to the fortress from within the Deep was made possible by a long stair which led to the Hornburg's rear gate.
In The Two Towers, the fortress guarding Helm's Deep becomes the refuge of some of the Rohirrim, under King Théoden, and the Battle of the Hornburg was fought there. Théoden said that "Helm's Deep would never fall while men defended it," giving an impression of invulnerability. This is then overturned, as an army Uruk-hai sent by Saruman later almost overwhelm the defences of the Rohirrim. This was achieved by the orcs using an explosive blast of fire within the culvert which, when detonated, destroyed the main defences. The origin of this explosion is unclear and Tolkien denotes it as "Saruman's devilry."
Aglarond and Angrenost (which later became Isengard) were the two fortresses built by Gondor to guard the approaches to 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 the care of the Rohirrim, 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 became 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.
In Peter Jackson's The Lord of the Rings film trilogy the keep itself, built into the mountainside, resembles a World War I bunker, in keeping with Tolkien's history as a soldier in the First World War. The entrance to the glittering caves of Aglarond is also contained within the Hornburg itself, rather than at the top of the deep behind the deeping wall as in the book. The Uruk-hai march up the ramp leading to the main gateway in a formation which resembled the Roman testudo, or locked-shields formation and the 'blasting fire' is depicted as gunpowder.
The fortress was featured as a scenario board in the Lord of the Rings board game.
The 2013 expansion to The Lord of the Rings Online entitled Helm's Deep depicts the fortress of Helm's Deep as well as the surrounding area of Western Rohan. The Battle of Helm's Deep features prominently in the expansion.
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