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|Realms||Firstly Gondor, then Rohan|
|Inhabitants||Gondorians, Later Rohirrim|
|Events||Battle of Helm's Deep|
|References||The Two Towers|
Helm's Deep was a large valley gorge in the north-western Ered Nimrais (White Mountains) below the Thrihyrne. It was actually the name of the whole defensive system including its major defensive structure, the Hornburg.
 Geography and description
The valley was blocked over its entire width by a trench and rampart called Helm's Dike and behind that lay the great fortress Hornburg. A large rampart known as the Deeping Wall blocked the valley; it consisted of solid rock except for a small culvert which allowed the Deeping-stream (a small stream) to pass. The stream flowed out of the Glittering Caves, a large cave system and the later site of Gimli's dwarf colony.
A long causeway 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.
Originally the valley was home to a small Gondorian regiment, but after Calenardhon was ceded to the Éothéod it became an encampment of the Rohirrim of the Westfold, and was used to guard the Gap of Rohan.
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 Long Winter of T.A. 2758–2759. Helm and his horn gave their names to the features of the Deep.
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. After the battle, the Huorns laid the dead Uruk-hai of Saruman in a large mound known afterwards as the Death Down. The Dunlendings were buried more honourably in a separate mound below the Dike.