Many of his historical books are centred on factual analysis of the Arthurian legend, and the archaeological past of King Arthur, beginning with his King Arthur's Avalon: The Story ofGlastonbury, in 1957. The book was inspired by what Ashe had read in G. K. Chesterton's Short History of England. He is a major proponent of the theory that the historical King Arthur was Riothamus, presented in an article in Speculum, April 1981, and expanded in The Discovery of King Arthur and in various further articles. His fresh idea was to scrutinise Arthur's foreign campaigns in Geoffrey of Monmouth's account and take the material seriously, concluding that, though the legendary Arthur is a composite figure, the career of Riothamus seems to underlie at least a major portion of Geoffrey's account, for which Ashe adduces passages in a Breton text and several chronicles. Ashe, co-founder and Secretary of the CamelotResearch Committee has also helped demonstrate, through a dig directed by Leslie Alcock in 1966–70, that Cadbury Castle, identified as Camelot by the 16th-century antiquaryJohn Leland, was actually refortified in the latter part of the fifth century, in works as yet unparalleled elsewhere in Britain at the time. Ashe's point is that when Leland picked out this hill as Camelot, he picked what seems to be the most plausible candidate; yet even an archaeologist could not have guessed that the fifth-century fortification was embedded in the earthworks, just by looking without digging. "I would say there must have been a tradition about the hill and its powerful overlord, handed down from the Dark Ages", Ashe has said, and added "In the film of the musical Camelot, you have a brief glimpse of a map of Britain, and Camelot is in Somerset. It's there because I told Warner Brothers to put it there. That is my one contribution to Hollywood." He has offered later mentions of Artoriani or "Arthur's men," a group of soldiers sharing Arthur's name that survived his death, as possible basis for the legendary Knights of the Round Table. Ashe is the author of a novel about an occult group that meets near the site of Avalon entitled The Finger and the Moon. He became a Fellow of the Royal Society of Literature in 1963. Ashe was appointed a Member of the Order of the British Empire in the 2012 New Year Honours for services to heritage. In 2015, Ashe was admitted by Glastonbury Town Council as an Honorary Freeman of Glastonbury "in recognition of his eminent services to the place as an author and cultural historian." Ashe's book Camelot and the Vision of Albion was often cited by Paul Weller as a major influence on The Jam's 1980 album Sound Affects.
