What is the name of the great fortress of Rohan?

Helm’s Deep is the great fortress and defensive refuge of Rohan. More precisely, Helm’s Deep is the valley and gorge in the White Mountains, while the fortress at its entrance is the Hornburg; the names are often used together in casual conversation.

The stronghold becomes central in The Two Towers when King Théoden leads Rohan’s people there to escape Saruman’s advancing army. The defenders withstand a much larger force of Uruk-hai, Orcs, and Dunlendings during the Battle of Helm’s Deep, also called the Battle of the Hornburg.

The location is named for Helm Hammerhand, an earlier king of Rohan. Its defenses include Helm’s Dike, the Deeping Wall, and the Hornburg itself. A common mix-up is calling the fortress “Helm’s Deep” without noting that the Hornburg is the actual castle, but Helm’s Deep is the canonical answer to this quiz’s wording.