The forces of Saruman, common Orcs and Uruk-hai, along with some orc-human hybrids (called "half-orcs and goblin-men" — which may have referred to or included the Uruk-hai themselves) and human Dunlendings, arrived at the valley of Helm's Deep in the middle of the night during a storm. Meanwhile, Legolas the Elf and Gimli the Dwarf agreed to compete, to see which one could kill the most orcs.
The attackers quickly scaled over the first defence, Helm's Dike, forcing the defenders there to fall back to the fortress. When the Orcs were close, the defenders drove them back with arrows and stones, but they managed to get close to the wall after multiple charges. They attempted to break down the gate with a battering ram, but a sortie led by Aragorn and Éomer scattered the forces.
The Orcs and Dunlendings then raised hundreds of ladders to scale the wall. Aragorn and Éomer repeatedly motivated the tired defenders to repel the Orcs coming up the ladders. However, some Orcs had crept in through a culvert which let a stream out of Helm's Deep, and while the defenders were busy with the assault on the wall, they were suddenly attacked from behind. This was repulsed and the culvert was blocked up under Gimli's supervision.
However, the enemies re-entered the culvert and blasted a wide hole in the wall using an ambiguous explosive device invented by Saruman, a "blasting-fire". The defenders then retreated to the Glittering Caves, Éomer and Gimli among them.
Soon Saruman's forces broke through the gate and gained entrance to the fortress. At this moment, however, the horn of King Helm was sounded, and a cavalry charge led by Théoden and Aragorn rode forth, followed by all the Rohirrim left inside. They cut their way through the Orcs and drove them back from the fortress walls.
Both armies then noticed that strange forest had suddenly sprung up (actually the arrival of many Huorns) which blocked the escape route for the Orcs. Then Gandalf arrived on Shadowfax, with Erkenbrand and a thousand infantry — the remaining strength of the Rohirrim that had been routed at the Fords of Isen. They charged into the fray. The Dunlendings were so terrified of Gandalf that most of them dropped their weapons. The surviving Orcs fled into the "forest" of Huorns, where they were completely annihilated.
After the battle, those Dunlendings who surrendered were given amnesty by Erkenbrand and allowed to return home (much to their surprise, since Saruman had told them that the men of Rohan would burn all survivors alive). The Rohirrim required that all hostilities cease, and that the Dunlendings retreat behind the River Isen again and never recross while bearing arms. Before they were freed, though, the Dunlending captives were put to work in repairing the fortress.
Among the Rohirrim dead was Háma, captain of Théoden's personal guard and doorward of his hall (he plays a significant role in the previous chapter, "The King of the Golden Hall"). Háma had fallen defending the gate and the Orcs had hewed his body after he died, an atrocity that Théoden did not forget during the upcoming parley with Saruman. Gimli was wounded, but had killed 42 to Legolas' 41.
The "forest" of Huorns had disappeared the next morning, and the Orcs had been buried in an earthen-works hill known as "Death's Down".
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