Helm's Deep was a large valley in the north-western Ered Nimrais (White Mountains). The valley was blocked over its entire width by the natural series of hills called Helm's Dike and behind that lay the fortress of Aglarond or the Hornburg, at the entrance to the Glittering Caves.
It was said in a myth that Helm's Deep would never fall while men defended it. No enemy had ever breached the Deeping wall, or set foot inside the Hornburg, but during the War of the Ring, Saruman's army of Half-Orcs managed to almost overwhelm the defences of the Rohirrim.
Gamling is left in charge of the forces defending Helm's Dike (a fortification across the Deeping-coomb before the stronghold of the Hornburg defending Helm's Deep) by command of Erkenbrand. About 1,000 men had been left to defend Helm's Dike, including Gamling's grandson, and Gamling notes that most of the defenders were either very old (like himself) or very young (like his grandson).
Half-Orcs were a new breed of Orcs, a cross with dunlendings to make them bigger and stronger, with more intelligence and a resistance to sunlight (at least Saruman's varient). Saruman armed them with different armour and weapons than those under the trall of Sauron. Saruman's symbol was the white hand on a black field on the shields.