In Geoffrey of Monmouth (HRB ii.9), Kaerguenit is the old name for Winchester; the Brut has Kaer Wynt. Linguistically, there is no problem here. The Romano-British name for Winchester was Venta Belgarum, and the first part of the name derives from a Brittonic *uentā, which , despite its obscurity, appears to mean something like ‘field’ or ‘market’. Following the processes by which Brittonic became Old Welsh, initial u- became gu-, later gw-, which in Middle and Modern Welsh mutates into w- following certain sounds (including final -r). We would therefore expect to find Romano-British Venta represented as *Guent or Cair Guent in Old Welsh. This is certainly the case with Caerwent in South Wales (and the kingdom of Gwent derives from exactly the same word, but without the mutation). In the Historia Brittonum Chapter 66a, we find Cair Guent, which could be any of the three Romano-British cities named Venta (Caerwent, Winchester or Caistor St Edmund), and there is no certainty that it refers to Caerwent. A second name, Cair Guintguic, derives from an otherwise unrecorded Brittonic *Uentouicion, which would represent a colloquial Latin *Ventouicium, ‘Venta town’. This is usually assumed (on very poor grounds) to have been Winchester: it could equally be Caerwent!
Nevertheless, they suggest that there is other evidence to shift the identification of Geoffrey’s Kaerguenit away from Winchester. They refer to the Brut’s mention of St Julianus (Sulien in Welsh) as Bishop of Kaer Wynt, the battle of Maes Vrien and the monastery of St Amphibalus. Taking these details in reverse order, St Amphibalus and his monastery are easily disposed of. The saint – like numerous medieval saints – never existed, but derived from a misunderstanding in a Life of St Alban, in which the saint’s cloak (αμφιβάλον in Greek and amphibalum in late Latin) became transformed into a person by a scribe who did not understand the rare word of Greek origin in his exemplar. If Amphibalus did not exist, nor did his monastery. In the Historia Regum Brittanie (vi.5), there is a church dedicated to the saint in Winchester; this is unhistorical, no matter where we choose to locate Kaer Wynt.
Secondly, the Battle of Maes Vrien in the Brut has its counterpart (as is to be expected) in Geoffrey of Monmouth (HRB v.8), who calls it Maisuria. For what it is worth, the spelling mais- in Geoffrey is an older form than the Maes of the Brut, but this is probably not significant. Accepting an equation of the eponym of the placename with Urien, king of Rheged, does not help them a great deal, as Rheged appears to have lain around Carlisle and the Eden estuary, no matter what Dee valley folklore (which they use as confirmation) has to say about Urien.
Sulien is found in two medieval Welsh genealogical tracts, the Bonedd y Saint and Achau’r Sant, neither of which is earlier than the thirteenth century. Achau’r Sant makes Sulien son of Cynwyd Cynwydion, which is certainly an error; Bonedd y Saint does not give his parentage, but makes him a saint of Lydaw, which is usually identified with Brittany. Even if we follow Blake and Lloyd in identifying Llydaw with Cornwall (which I believe to be mistaken), they do not use this to identify the site of Kaer Wynt.
Instead, they settle on Corwen, a village west of Llangollen, in the upper Dee valley. Ss Sulien and Mael are the patrons of the church there, which allows them to suggest that the bishopric of Julianus was based here. However, there can be no connection between the modern placename and Kaer Wynt, as Corwen was formerly called Corfaen, meaning ‘sacred stone’. However, they do hint that the nearby hillfort of Caer Drewyn may preserve the name (their amateur etymologising makes it “town of Wyn or maybe Wynt”, whereas Drewyn is quite reasonably analysed ‘white town’, perhaps a reference to the local limestone).
There is nothing, then, in the material they assemble that suggests that their alternative location is even a contender for the identification of Kaer Wynt. Changes of identification of this scale require robust evidence, not later traditions. The fact remains that the Brutiau depend on Geoffrey of Monmouth’s Latin Historia Regum Brittanie, which explicitly states that Kaerguenit>Kaer Wynt is Winchester: there is no ‘mistranslation’.