What we know today as the West Country - Cornwall, Devon, Somerset and Dorset - was once covered by a vast sub-Roman kingdom called Dumnonia. The name derives from the Celtic Iron Age & Roman-British tribe of the Dumnonii who lived in the two latter counties and the western part of Somerset. The name is retained today in Devon, the Saxon modern version derived from Defnas (the men of Devon) via the late-Celtic form, Dyfneint. 

However, it is Cornwall, in the far west, which is usually thought of as the heartland of Celtic survival in this region. It appears to have been an area of semi-independence at times and, in later centuries, was certainly the last remnant of Dumnonia to be overrun by Saxon invaders. It is interesting to speculate about the name itself.   Cornwall may derive from the Celtic tribe of the Cornovii. A people of this name are known, from Roman sources, to have lived in the Outer Powys to Shropshire area of the later Wales and England. John Morris suggests a contingent was sent to the West Country in order to rule the land there and keep out the invading A similar situation occurred in North Wales. However, there is no evidence for this move west, and Cornish placenames of a similar age indicate that there was an independent tribe of Cornovii in the West Country. Corn is a common element in British place-name etymology, literally meaning Horn, but in this context a horn-shaped peninsula. It is the ideal description for Cornwall. The original name was Cerniw. The suffix is the same as the Saxon word Welsh, meaning foreign.

The Kings of Dumnonia, like their Saxon successors, were, no doubt, constantly on the move.  One of their main Royal residences, perhaps a "Capital" of sorts,Cadbury Castle in Somerset,  Somerset, probably named after the sixth century King Cado. Other important centres included Dunster and Tintagel. The status of these places may have changed over time. The latter, for instance, being very exposed, was probably a Summer residence only, perhaps sometimes left in the care of governors or duces like the legendary, Gorlois. At other times, it may have been the capital of the sub-kingdom of Cornwall. There were a number of other such kingdoms extant at various times in Dumnonia, though details are often obscure. Sub-division of the Kingdom followed the traditional split between sons. This was certainly the case with Cornwall and, possibly, the legendary Lyonesse, centred on the Scilly Isles. Other regions were taken over by exiled Royalty from elsewhere, seeking a new power-base, forcibly or otherwise  A little known kingdom, centred on the Hayle estuary, on the Penwith peninsula thus came under the control of King Tewdwr Mawr of Brittany; whilst a dynasty from Staffordshire established the sub-Kingdom of Glastening around Glastonbury in Somerset. Other regions on the eastern borders may have been completely independent of Dumnonia. Like the Kings of Caer-Baddan (Bath), the last of whom fell at the Battle of Dyrham in AD 577, or the otherwise unknown lords who have left ogham inscribed memorials at Wareham in Dorset.
There were a number of other such kingdoms extant at various times in Dumnonia, though details are often obscure. Sub-division of the Kingdom followed the traditional split between sons. This was certainly the case with Cornwall and, possibly, the legendary Lyonesse, centred on the Scilly Isles. Other regions were taken over by exiled Royalty from elsewhere, seeking a new power-base, forcibly or otherwise  A little known kingdom, centred on the Hayle estuary, on the Penwith peninsula thus came under the control of King Tewdwr Mawr of Brittany; whilst a dynasty from Staffordshire established the sub-Kingdom of Glastening around Glastonbury in Somerset. Other regions on the eastern borders may have been completely independent of Dumnonia. Like the Kings of Caer-Baddan (Bath), the last of whom fell at the Battle of Dyrham in AD 577, or the otherwise unknown lords who have left ogham inscribed memorials at Wareham in Dorset.