The Hornburg was a great Gondorian fortress later used by the Rohirrim.
It was said among the Rohirrim that no enemy had ever taken the Hornburg by assault and it was considered the strongest fortress in Rohan, stronger than Edoras and Dunharrow.
Geography
The fortress was built by Gondor near the western border of its province of Calenardhon on a stone promontory, later called the Hornrock by the Rohirrim, at one end of the Deeping Wall. This wall blocked the entrance to a deep valley, later called by the Rohirrim Helm's Deep, south-east of the northern end of the White Mountains, below the peaks known as the Thrihyrne. It guarded the southern end of the Gap of Rohan.
The fortress was built by Gondorians, at a date not known to the Rohirrim, who said that it was constructed in "far-off days of the glory of Gondor [by] the sea-kings ... with the hands of giants". It was originally called Aglarond, a name used also for the caves located at the head of the valley.
In later years whole regions of Gondor, including Calenardhon, were depopulated or evacuated. Steward Cirion gave the province to the Rohirrim as a reward for their assistance at the Battle of the Field of Celebrant in T.A. 2510. Masons of Gondor helped to repair the fortress of Aglarond. After that it was handed over to the Rohirrim and the old hereditary Gondorian guards of Aglarond were stationed at the fortress of Angrenost (called Isengard by the Rohirrim). The Rohirrim called the fortress of Aglarond the Súthburg, probably because it guarded the southern end of the Gap of Rohan, as Isengard guarded the northern end.
When Wulf and the Dunlendings overran Rohan, Helm Hammerhand, ninth King of Rohan, sought refuge in the fortress during the Long Winter of T.A. 2759 where he was besieged. The fortress was called the Hornburg after the days of King Helm, because of Helm's great horn, which he had blown before his lone raids into the camp of the enemies during the siege and because a horn sounded from its tower echoed in the Deep behind.
During the period Battles of the Fords of Isen leading up to the War of the Ring, Erkenbrand, the Lord of Deeping-coomb and of much other land in Westfold, resided at the Hornburg.