Few Roman remains have been found in Cornwall, and there is little evidence that the Romans settled or had much military presence there. After the collapse of the Roman Empire, Cornwall (along with Devon, parts of Dorset and Somerset, and the Scilly Isles) was a part of the Brittonic kingdom of Dumnonia, ruled by chieftains of the Cornovii who may have included figures regarded as semi-historical or legendary, such as King Mark of Cornwall and King Arthur, evidenced by folklore traditions derived from the "Historia Regum Britanniae". The Cornovii division of the Dumnonii tribe were separated from their fellow Brythons of Wales after the Battle of Deorham in 577 AD, and often came into conflict with the expanding English kingdom of Wessex. The regions of Dumnonia outside of Cornwall (and Dartmoor) had been annexed by the English by 838 AD. King Athelstan in 936 AD set the boundary between the English and Cornish at the high water mark of the eastern bank of the River Tamar. From the Early Middle Ages, language and culture were shared by Brythons trading across both sides of the Channel, resulting in the corresponding high medieval Breton kingdoms of Domnonée and Cornouaille and the Celtic Christianity common to both areas.