The name Dumnonia connects the early medieval south-west with the Dumnonii of Roman Britain. Exeter, known as Isca Dumnoniorum, was an earlier regional centre, but that does not establish a single permanent capital for every later ruler. The extent and organisation of post-Roman authority remain difficult to reconstruct. Devon and Cornwall form its principal geographical setting; connections with neighbouring districts are less easily translated into fixed boundaries. Older hillforts, coastal settlements and the sea routes around the peninsula all belong to this setting, rather than to a simple story in which one unchanged Roman province immediately became a fully documented kingdom.

Gildas provides an important early written reference when he attacks Constantine of Damnonia in The Ruin of Britain. He accuses the ruler of killing two royal youths in a religious setting after making solemn promises. This is a hostile moral denunciation, not an impartial royal biography, but it is evidence for a named ruler in Gildas's own world. Centuries later, Geoffrey of Monmouth retells a related pattern of church killings when Arthur's successor Constantine destroys Mordred's sons. The later story names Cador as Constantine's father. These accounts should be compared without assuming that every detail of the romance belongs to the earlier king.

Welsh tradition supplies further links through Geraint son of Erbin. The poem associated with Llongborth remembers Geraint in a battle setting that also invokes Arthur's warriors, while the romance Geraint and Enid places him in a richly developed Arthurian household. Neither text alone authenticates Arthur's presence at a historical battle. Genealogies involving Erbin, Custennin and Cador vary, and repeated royal names need not identify one person. In particular, the Gerent or Geraint associated with conflict against Wessex in 710 belongs to a much later period than the customary setting of Arthur's reign.

Pressure from Wessex changed the political geography of the south-west over several centuries. The Anglo-Saxon Chronicle records fighting involving Ine and a British king in 710, while later entries describe campaigns in the western peninsula. Such notices do not provide a complete history of conquest or a single unquestionable date on which every part of Dumnonia ceased to be distinct. Cornwall retained a separate identity as power shifted. The ninth-century king Dungarth, also associated with the name Doniert, is recorded as having drowned; the annal does not establish the elaborate story of an Irish execution for collaboration found in some retellings.

At Tintagel, buildings, imported Mediterranean pottery and glass provide a firmer material basis for understanding the period. English Heritage interprets the early settlement as a probable stronghold used by Dumnonian rulers, one of several possible royal centres rather than a proven capital. Tin is a plausible part of the exchange that brought luxury goods across the sea. South Cadbury's substantial post-Roman defences likewise demonstrate organised activity without identifying their builder as Arthur. Dumnonia's importance lies in this combination of regional power, maritime contact and later storytelling: a historical landscape that supplied medieval writers with names and places, not archaeological confirmation of an entire legendary court.