The fortress at Helms Deep in Ered Nimrais. Hornburg was built upon Hornrock where Helms Deep widened out into the Deeping Coomb. It consisted of a tower inside two round courtyards each one with a high wall. The outer courtyard was not perfectly round, it had a at the Southern side an angle where the wall continued out into the Deeping Wall, closing off Helms Gate, Helms Deep and furthest behind the Glittering Caves. The North-Western side of the fortress was built up against the Northern face of the steep mountainwall. Around the feet of Hornburg and the Hornrock from the Deeps South-East ran the Deeping Stream. There were two gates in the outer wall, the reargate leading by a stair into the Deeps and the great arched wooden front gate leading by a ramp and a causeway over the Deeping Stream out into the Deeping Coomb and by a road alongside the stream to Helms Dike. Besides the rear and frontgate a small postern door near the mountainwall lead by a narrow path round the wall to the front gate. From the outer court a single gate located to the North of the inner wall lead into the inner court where the tower stood. The outer wall had battlements from where men could look out and shot at attackers and four flight of stairs lead up to the battlement. A fifth stair lead from the angle of the outer court up to the Deeping Wall.

Hornburg was build in the first days after Gondor was established by the Dúnedains.