Helm's Deep was a valley in the north-western White Mountains of Middle-earth. Helm's Deep, with its fortress the Hornburg, becomes the refuge of some of the army of Rohan, the Rohirrim, under King Théoden, from assault by the forces of Saruman. Although Théoden says that the Hornburg has never fallen to assault, in the battle a massive army of Uruk-hai and Dunlendings sent by Saruman almost overwhelms the defences. Saruman's Orcs breach the fortress wall that blocks the valley by setting off an explosion in a culvert; Aragorn names it Saruman's devilry and the fire of Orthanc; the critic Tom Shippey calls it a kind of gunpowder. The defenders hold out in the fortress until dawn, when Théoden and Aragorn lead a cavalry charge that drives the Orcs from the fortress. They are surprised to see the valley to the enemy's rear blocked by a forest of tree-like Huorns that have walked from Fangorn in the night. On the side of the valley are relieving forces assembled by Gandalf and Erkenbrand, a Rohirrim leader. These attack, driving the Orcs into the angry Huorn forest, from which the Orcs never emerge; the Huorns bury the Orcs's bodies in an earthen mound known as Death's Down.