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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 the trench and rampart called Helm's Dike and behind that lay the fortress Súthburg, which extended to the Deeping Wall built across the entrance to the Glittering Caves and consisted of solid rock except for a small culvert which allowed water from the Deeping-stream (coming out from the Deep) to pass; this rendered a fresh supply in sieges of great length.
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.
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.