The valley was 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. A long causeway wound up to the great gate of the fortress itself. Inside the keep there were stables and an armory, as well as a great hall in the rear which was dug out of the mountainside. There also was a great tower the top of which consisted of the great horn of Helm Hammerhand. The Deep, which stood next to the fortress, was barred by the long Deeping Wall, which consisted 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, though this small weakness would eventually prove to be the wall's undoing. Access to the fortress from within the Deep was made possible by a long stair which led to the Hornburg's rear gate, where there was a massive system of caves behind the fortress.
Helm's Deep was the largest fortress of Rohan, where the guards of the Fords of Isen lived. Originally named Aglarond, it was, with Angrenost (Isengard), one of two Gondorian strongholds built to guard the Fords of Isen.
During the War of the Ring, the fortress guarding Helm's Deep again became the refuge of some of the Rohirrim, now under King Théoden, and the Battle of the Hornburg was fought there. It was during this battle that the Deeping Wall was destroyed by Uruk-hai explosives.
The Walls are essentially the area of Helm's Deep that surrounds the entire fortress, which is used for reflecting enemy attacks during a siege. The Walls are made of solid rock and are almost completely impenetrable. However, there is one part of the Wall that is extremely vulnerable, which is only a drain.
The Armory, or Armoury, is an area of Helm's Deep that is used to store, clean and repair weapons that the Soldiers use to protect the fortress. In the preparation for protecting the fortress against a full-scale siege, soldiers go to the Armory to get their weapons to defend the fortress.
The Causeway is a long section connected to the fortress of Helm's Deep, used as a road to separate the fortress from the ground. Soldiers can easily move from the land onto the Causeway, which will lead them to the Keep. During the Battle for Helm's Deep, the Orcs and the Uruk-hai used the causeway to breach the Keep to effectively take over the fortress.
The Caves aren't actually a part of Helm's Deep, but are a series of large hollow areas inside the mountain. A path is connected from Helm's Deep to the Caves, which is usually used for keeping people who cannot fight, such as women and children, during a siege.
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