Helm's Deep is based on the Cheddar Gorge, a limestone gorge deep in the Mendip Hills, with a large cave complex that Tolkien visited on his honeymoon in 1916 and revisited in 1940, and which he acknowledged as the origin of the Glittering Caves of Aglarond at the head of Helm's Deep, behind the fortress.
Helm's Deep is based on the Cheddar Gorge a limestone gorge deep in the Mendip Hills with a large cave complex that Tolkien visited on his honeymoon in 1916 and revisited in 1940 and which he acknowledged as the origin of the Glittering Caves of Aglarond at the head of Helm's Deep behind the fortress