From Wikipedia: Battle of the Hornburg

 

Strength

 

In the book, the size of the force at Helm's Deep for Rohan was nearly 1000, but many more were coming into the fort from across Rohan. The estimated number of total Rohan defenders could've been as much as 1,700 by the time Saruman's army arrived. Pippin Took says that the force that left Isengard numbered 10,000 at least, most marching towards Helm's Deep and others heading off to unknown locations. Though the battle appears severly lopsided, as Uruk-hai were much better in battle than simple orcs, the defenders managed to hold the fort until a force of nearly 2,000 men led by Gandalf along with a forest of Huorns arrive at dawn and surround the Uruks.

 

Background

 

After being released from his malevolent adviser Gríma Wormtongue's influence by the benevolent Wizard Gandalf, Théoden set out to the Fords of Isen, where his marshal Erkenbrand was fighting Saruman's forces. However, Théoden found out that they had been scattered. Gandalf advised him to take refuge in the Hornburg fortress of Helm's Deep, an area named after one of their kings. Then Gandalf left on an unexplained errand. Théoden's army went to the area, where local people were commanded by a captain called Gamling the Old. Many of the forces there were too old or too young. The women and children of Théoden's capital Edoras were safe in Dunharrow, led by the King's niece Éowyn.

 

The battle

 

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 reentered 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 broke free.

 

Both armies then noticed that many Huorns had moved to block a possible 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 they could no longer fight. The Orcs lost control and ran into the "forest" of Huorns, where they were completely annihilated by the ents.

 

After the battle, those Dunlendings who surrendered were given amnesty by King Théoden 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.

 

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"). Gimli was wounded, but had killed 42 to Legolas' 41.