Helm's Deep has been a fortress for thousands of years, built sometime during the 2nd age at the same time as Isengard, the two having the purpose of protecting the Gap of Rohan from the enemies of Gondor. Originally guarded by a strong and constant regiment, Helm's Deep feel partially into disarray as the Kingdom of Gondor also fell in might and honor. After the Great Plague of the Third Age and an attack by the Easterlings, Helm's Deep was taken over by the Rohirric due to their efforts (refer to the Preview of Edoras for more information).
Helm's Deep is located in the Southwestern part of the White Mountains, a few days ride from Edoras and Isengard. During the Dunleanding Invasion of the Third age, the Rohirric used Helm's Deep as their fortress before they could strike back and re-take their lands. In the War of the Rings, Helm's Deep once again proved itself, as an army a third of the size of the enemy (about 3000-4000 vs. more than 10,000 Orcs) won a significant victory destroying Saruman's base at Isengard, with the help of the Ents of course.
Helm's Deep maintains a small unit of about a thousand of Rohirric Soldiers, an amount that remains constant throughout its history. It now additionally has about 1000-2000 dwarves (part of the Durin (Thorin) Line) living in the Glittering Caves, called for their walls glitter with crystals and gems. Led by Gimli, the caves are quickly becoming one of the key towns of Dwarfdom. Though the Caves are technically owned by Eomer still, Gimli is allowed to do as he pleases. It is said that the caves are one of the most beautiful places in all of Middle Earth.
Helm's Deep thus is becoming one of the gems of the New Fourth Age Order, as dwarves and men live together to further increase the glory of the realm.
