The Historia Britonnum battle list says King Arthur’s first battle was at the mouth of the river called Glein. There is no definite answer to where this river might have been. If the river stil bears that name, philologists say it can only be either the River Glen in Northumberland or the River Glen in Lincolnshire. Fitzpatrick-Matthews says the name Glein implies a root Glano- meaning ‘pure’ or ‘clean’.
Culturally, it would be realistic to expect the Glein, like King Arthur’s next four battles, to have been on or near a boundary of Lindinis. There are no river names relevant as either possible derivatives or possible translations of Glano- but there is a place-name which could just possibly be the right one: Clannaborough.
Clannaborough is in Devon, ten miles from Cadbury Castle near Tiverton, a hillfort that could well have guarded part of the western boundary of Lindinis. Cadbury Castle is the most westerly of the four hillforts named after Arthur’s predecessor Cador. There is also a place called Cadbury Barton nine miles north of Clannaborough. In conjunction with Cadbury Castle and nearby Cadeleigh (= Cador’s Wood), these names point to the northern half of the western boundary of Lindinis in Cador’s time – primarily the third quarter of the fifth century – having been the River Taw.
The website for nearby Colebrooke – http://turnertree.net/colebrooke.html – says the area was thickly inhabited in Roman times (which would be in stark contrast to now!).
Clannaborough is no more than a highly speculative guess for Arthur’s Glein battle. The Domesday spelling Cloenesberg (and variants) has been translated as ‘cloven hill’, but no reason has been proffered as to why the hill there would have been called ‘cloven’. It is possible that the locality was known to its Celtic inhabitants as Glano- and that when the English conquered this part of Devon (possibly as early as 580, according to Hopkins) they simply heard ‘Glan’ as ‘Cloen’. But there can be no certainty that this was so.