The Battle ot Helm's Deep

The Battle of the Hornburg, also known as Battle of Helm's Deep, was the first grand-scale battle of the War of the Ring where the Rohirrim under King Theoden defended the Hornburg from Saruman's army of Uruk-hai.

History
After being freed by Gandalf from Gríma Wormtongue's influence, Théoden went to the Fords of Isen where Erkenbrand was battling Saruman's orcs. When Théoden got there, he realized that the army was scattered, so Gandalf told Théoden to help the refugees and send them to the Hornburg where many people from Westfold were also taking refuge. Soon Gandalf brought the Rohirrim to defend Helm's Deep from Saruman's attacking forces. A local captain called Gamling the Old led the soldiers. Unfortunately, according to Gamling, "The soldiers have seen either too many winters, or too few."

The Battle
The army of Saruman was reinforced by the way due to casualties in the battles of the Fords of Isen it was made up of Uruk-hai, Orcs, Wild Men of Dunland and Half-Orcs and they arrived in the middle of the night.
The Orcs quickly overran the first defence of Helm's Dike, but sustained many losses in it "filling up the dike to the top". After moving past the Dike the orcs arpproached the fortress of Hornburg they sent arrows into the fortress. However they did not get any response. But when they got closer they were met with a volley of arrows and rocks which made them break ranks. The Orcs soon got close enough to Hornburg and attempted to break the gate down with tree trunks, but were scattered by a small attack team led by Aragorn and Éomer from a postern-door that ran between the brink of the cliff and the wall of the Hornburg.
After the team had driven back the Orcs with the ram, the team retreated from the new assault upon the gate. While running back, two Orcs who had faked their deaths knocked Éomer to the ground. As Aragorn ran back to save him, a hidden Gimli hewed the Orc's heads saving Éomer and giving Gimli his first two kills to Legolas's twenty (seventeen in film version).
The attackers then raised hundreds of ladders to scale the Deeping Wall; the worn-out defenders threw down the ladder attacks, but were attacked from behind by Orcs who had crawled through the culvert at the base of the wall. Gimli and Gamling the Old with a unit of men from the Westfold attacked the orcs who got through the hole. Gimli, after getting twenty-one kills to Legolas's twenty-four, helped block the culvert with stones flooding the stream. Unfortunately, this blockage was destroyed along with a good portion of the Deeping Wall by Saruman's devilry; the fire of Orthanc (most likely gunpowder which the Númenóreans probably knew about but their secret was probably taken by Sauron, and then by Saruman).
Out from the new larger hole a huge host of Orcs poured in, and after a desperate battle in the Deep, the defenders led by Éomer, Gimli and Gamling retreated into the Glittering Caves while Aragorn and Legolas fought their way into the keep via a large stairway from the Deep to the citadel.
During the retreat, Aragorn was overcome by Orcs; but was saved by Legolas and a boulder that was cast down upon the Orcs from the wall. Aragorn called for a parley and told the orcs that they would die if they didn't surrender, but the Uruk-hai laughed at his threat and attempted to shoot Aragorn down, almost killing him. The Orcs then made it to the gate and destroyed the arch with the 'blasting-fire'. Suddenly, Théoden and Aragorn led a vicious cavalry charge out of the keep to the blasts of the great horn of Helm, followed by the defenders of the Hornburg, driving the Uruks away from the remains of the gate. At the same time, Éomer and Gimli with the men from the caves pushed the orcs out of the Deep. The army of the Mark drove the Enemy behind the Dike into the Deeping-Coomb.
Just then, Gandalf on his horse Shadowfax arrived with Erkenbrand and a thousand swordsmen on foot, and they charged the Orcs, trapping them between Erkenbrand's army, Theoden's army and the Huorns of Fangorn and so Saruman's army was defeated. After the orcs and Uruks were driven into the Huorns, the Dunlendings surrendered and were spared, but were forced to take an oath never to attack Rohan again. None of the Orcs driven into the Huorn forest were ever seen again. The Orcs that survived, if any, made for the plains.

Immediate Aftermath
Less than half of the 10,000 Uruk-hai survived the main battle, after being driven into Huorns at most one fourth managed to escape into the plains of Rohan. The surviving Dunlendings were sent home and their slain were buried in a communal mound. The orc carcasses were simply piled off to the side of the battlefield and left to rot. One of the Rohirrim dead was Háma, a captain of Théoden's personal guard and doorman of Meduseld. Gimli had lost his helmet in the battle and was wounded badly on the head, but he had killed forty-two orcs to Legolas' forty-one (forty-three to forty-two in the movie version).

Later Aftermath
The battle was critical to the survival of Rohan and their participation in the War of the Ring as a whole; had the Rohirrim lost the Battle of the Hornburg, their nation as a whole would have been obliterated by the rampaging legions of Saruman's livid Uruk-hai. Merry and Pippin's persuasion of Treebeard to attack Isengard was critical as well, and guaranteed Rohan's survival for a time. Had the battle been lost, and the Ents refused to attack Isengard, Saruman's forces would've opened up a two-front war with Gondor and crush them. This battle also gave valuable, albeit painful, war experience to the hitherto-lax Riders of Rohan that would prove to be invaluable on the Pelennor Fields but a short while after.