His conquest of Northumbria, which he held for a year or two after Edwin died, made him one of the last recorded Celtic Britons to hold substantial territory in eastern Britain until the rise of the Welsh House of Tudor.

[3][4] Historian Alex Woolf, however, presents the case that the genealogists have erroneously inserted Bede's Cadwallon into the pedigree of the unrelated Kings of Gwynedd as son of Cadfan.

Instead, Woolf suggests that Bede's Cadwallon was the Catguallaun liu found in genealogies as son of Guitcun and grandson of Sawyl Penuchel, rulers in the Hen Ogledd or Brythonic-speaking area of northern Britain.

They refer to a battle at Digoll (Long Mountain) and mention that Cadwallon spent time in Ireland before returning to Britain to defeat Edwin.

From there, according to Geoffrey, Cadwallon led an army into Dumnonia, where he encountered and defeated the Mercians besieging Exeter, and forced their king, Penda of Mercia, into an alliance.

"[11] Furthermore, Bede tells us that Cadwallon, "though he bore the name and professed himself a Christian, was so barbarous in his disposition and behaviour, that he neither spared the female sex, nor the innocent age of children, but with savage cruelty put them to tormenting deaths, ravaging all their country for a long time, and resolving to cut off all the race of the English within the borders of Britain.