The Battle of the Hornburg took place on March 3-4, T.A. 3019 after the Battles of the Fords of Isen.
Many wolf-riders were already in the Westfold Dale and the Deeping Coomb when théoden’s Riders arrived. The Rohirrim went up to the Hornburg. After the rearguard retreated from the Dike, the attackers poured through.
Éomer arrayed most of the men on the Deeping Wall and its tower, “where the defence seemed more doubtful.” After volleys of arrows, the attackers surged forward in an attempt to scale the Wall. At the gate of the Hornburg “the hugest Orcs were mustered, and the wild men of the Dunland fells.” Using two great battering rams, the enemies were splintering the wooden doors. Aragorn, Éomer, Gimli, and a handful of warriors exited from a postern-door and threw the enemies from the Rock; then they instead directed a barricade to be built to hold the gate.
As Saruman’s forces were unsuccessful at the gate, they quietly crept in through the culvert in the Deeping Wall but were spotted and slain or driven into the gorge, falling to defenders farther up the Deep. Gimli directed som of the men in blocking the culvert; but later, when the fight on the Wall was the fiercest, the Orcs blasted the culvert open just as many other enemies managed to scale the Deeping Wall. All “the defense was swept away,” and the defenders retreated to the fastness of the Burg and the caves: Aragorn, Legolas, the Men of the king’s guard, and many others gained entrance to the Citadel, while Éomer and Gimli fought their way back to the Glittering Caves. The assaults continued throughout the night, with the attackers able to move from all sides of the Hornburg except next to the mountain. Still they were unable to break through the Hornburg gate until dawn, when they blasted an opening.
Instead of Orcs pouring into the Hornburg, however, Théoden led forth his Riders, perhaps nine hundred or so if about half the surviving defenders had escaped to the caves. So powerful and sudden was their onset that Saruman’s forces (though far greater in number despite casualties) were driven in rout back to Helm’s Dike. Only a quarter mile beyond the dike was a wood of Huorn’s, however, and the enemy milled between. Suddenly Gandalf and Erkenbrand appeared on the western ridge. Trapped between unscallable cliffs to the east and attckers both west and south, the Dunlendings surrendered, and the Orcs ran into the “wood” of Huorns from which none escaped. The men from the caves arrived too late – the Battle of the Hornburg was already won and all of Saruman’s great host destroyed.
Rohan and Allies:
Théoden was said to have brought more than a thousand Riders out of Edoras, initially to ward off new attacks from Isengard against the Fords of Isen. After hearing of the disastrous Second Battle of the Fords of Isen, they turned aside towards Helm's Deep instead. Gamling the Old was already there with approximately 1000 foot-soldiers. Thus, the total number of defenders on March 3 were about 2000.
On the morning of March 4, Gandalf and Erkenbrand arrived with a thousand more infantry, and a wood of Huorns that according to Merry counted "hundreds and hundreds."
The host of Saruman:
The most precise account of their numbers is probably given by Merry, who claimed that there were "ten thousand at the very least." The host of Saruman consisted of Orcs, Uruk-hai, Wargs and Dunlendings.