HornburgThe Hornburg - The fortress of Rohan.

 

"At Helm's Gate, before the mouth of the Deep, there was a heel of rock thrust outward by the northern cliff.  There upon its spur stood high walls of ancient stone, and within them was a lofty tower.  Men said that in the far-off days of the glory of Gondor the sea-kings had built here this fastness with the hands of giants.  The Hornburg it was called, for a trumpet sounded upon the tower echoed in the Deep behind, as if armies long-forgotten were issuing to war from caves beneath the hills.  A wall, too, the men of old had made from the Hornburg to the southern cliff, barring the entrance to the gorge.  Beneath it by a wide culvert the Deeping-stream passed out." ~The Two Towers, Book III, chap. 7 (Helm's Deep)