About Constantine of Dumnonia (map Cador)
Contemporary of Gildas (c. 500-c. 570), who addressed a section of De excidio to him in severest terms. (Some traditions have it that Gildas' admonitions were so effective that Constantine abdicated and entered a monastery.)
This large and well-founded kingdom took in all of Cornwall (Cornubia), Devon (Dyfneint), and much of Somerset (the 'Summer Land' of the Mabinogion). It apparently emerged much earlier than many of its peers, perhaps up to two hundred years earlier than equivalents in England, while in Wales the picture is more confused. However, much of this claim to early independent or semi-independent kingship may be later tradition alone, perhaps based on an oral tradition that expanded the standing of a line of prominent native administrators. One of the curiosities of later Roman Britain is the appearance of stones recording building or repair work on Hadrian's Wall. The stones are undated, but are placed in the mid-fourth century and two record work by the civitas Dumnoniorum and the civitas Durotrigum. They seem to represent either an enforced labour party under military supervision (which would not discount the possible presence of a semi-independent Dumnonia), or the provision of civilian labour to maintain the country's defences (which would mitigate against semi-independence, but not wholly).
Dumnonia's original capital would have been Isca (late British Caer Penhuelgoit according to Geoffrey of Monmouth, modern Exeter). Archaeology confirms that this site was abandoned in the fifth century, and, given the advances of the West Seaxe, the capital would continually have been relocated to the west. The main portion of Dumnonia was formed by modern Devon (the Defnas Britons, thought to mean 'deep valley dweller' Britons, was more likely to be the result of a consonant shift whereby 'dumnon' became 'defnon' became 'devon'). Defnas fell to the West Seaxe between 652-685, while the Cornish remnant was still fully independent until 875.
Although under nominal Roman control until the late fourth century, the Dumnonii probably exercised a certain amount of self-government in their own lands. The Romans clearly found the Dumnonians to be fierce in their resistance to invasion, and it is thought that the two sides reached an understanding whereby the Dumnonians would be cooperative clients if the Romans left them alone. They most probably re-established their kingdom as a power in its own right by the time of Magnus Maximus, as the latter prepared Britain's defences prior to establishing his own claim for control of the Roman empire in AD 383, and Dumnonia was fully independent by 410, now also incorporating the former territory of the Durotriges.
(Additional information by Edward Dawson, from The Landscape of King Arthur, Geoffrey Ashe, and from Roman Britain: A New History, Guy de la Bédoyère.) http://www.historyfiles.co.uk/KingListsBritain/BritainDumnonia.htm