In the northern White Mountains lay a defensible valley and gorge, and when the Gondorians settled Calenardhon they fortified this valley. A great wall was built across its mouth, and a fortress keep was constructed to guard the wall, known as the fortress of Aglarond after the Glittering Caverns that lay within the gorge behind.

When the Rohirrim came south long afterwards to settle Rohan, the valley and its castle came into their keeping. They called the valley the Deeping-coomb, and at first they named the fortress the 'Súthburg', 'southern¹ fortress' in their own tongue. This name incorporates Old English 'burg', a general word for a fortified town or keep. The same element was kept when the fortress was renamed some centuries later as the 'Hornburg', after the great hero Helm Hammerhand and his famous horn. In both cases the word 'burg' was retained within the name, and the castle would at times be referred to simply as 'the Burg'.