Southern England is almost certainly the location of Badon and the "mount" may be in reference to an Iron Age hill-fort. There are many candidates for the site: Hills near Bath (favored by Geoffrey), Badbury rings in Dorset and Liddington Castle in Wiltshire and the close by town of Badbury. The Liddington site is the most promising with its earthen defenses known to have been strengthened about the right time.
While the battle is real, Arthur's connection with it is problematic. The Briton leader is not named by Gildas. Arthur is attributed this role first in the Historia Brittonum (9th century) and later in the Annales Cambriae (10th century). Both entries are questionable due to legendary flourishes contained within and the existence of more plausibly Arthurian battles 40-50 years earlier. While the early Welsh show little interest in Badon nor do they note it for the King's glorification (mention of it is absent from the triads), Bardic tradition may have introduced Arthur here due to the fact that Badon was the culmination of the war Arthur was associated with and the possible presence of "Arthur's men" - soldiers fighting under his banner and name.