The Castle (‘Castellum’) Guinnion has been identified with the Roman fort of Vinovium at Binchester, although the great Professor Kenneth Jackson thought this unlikely. It has since been noted, however, that Ptolemy’s alternative Vinnovium (B. *Uinnouion) brings us very close to the later name set down by Nennius. Vinnovium should have given in Old Welsh at this stage a form in –wy, but it could be that – ion has been maintained as a so-called ‘learned form’. Thus the identification should not be rejected.
Binchester is not far south of Hadrian’s Wall on the Roman Dere Street. The fort stands on a spur of high ground some one and a quarter miles north of Bishop Auckland. It overlooks a loop in the river Wear and is in an excellent defensive position.
The fort was built in 79 CE during the Roman advance into northern England. From the early second century Binchester and the other Dere Street forts became important supply depots for Hadrian’s Wall and developed as military centres controlling the region south of the Wall.
The fort was in continuous military use until the early years of the 5th century. After the final withdrawal of the garrison the fort and the surrounding vicus (civilian settlement) continued to be occupied by the local, native population and it would seem that Binchester remained an important small town. By the beginning of the 6th century the fort buildings were being torn down and stripped of stone. Part of the site was ut lized by Anglo-Saxons as a cemetery.
I had at one time proposed Carwinning in Dalry parish, Ayrshire, which is from a Caer + Winnian. This looks very good, but if a battle were fought here during Arthur’s time it was certainly not against the English.
There are, of course, several “Gwynion” place-names in Wales, but again, none of them work for Arthur.
In passing, I would put forward an additional, though tentative argument in support of Binchester as Guinnion. In the Introduction I alluded to Arthur’s carrying of the image of Mary on his shield during the Guinnion battle. It may not be a coincidence that Binchester is known for having a cult of the Mother Goddesses at its Roman fort. It is possible Mary in the Arthurian battle context is a Christian substitution for the Binchester ‘Mother.’
From http://theses.univ-lyon2.fr/documents/lyon2/2009/beck_n#p=0&a=top (Goddesses in Roman Religion, thesis by Noemie Beck, 2009):
“In Britain, the dedications to the Matres amount to approximately fifty inscriptions, all but a few from military sites, notably along Antonine’s and Hadrian’s Wall, and dedicated by soldiers. One of the few exceptions is the inscription to the Matres Ollototae, which is from the non-military site of Heronbridge, Cheshire. This suggests that the cult of the Matres and Matronae was brought to Britain by auxiliary troops from the Continent, such as by the Germanic legionaries of the Roman army. However, it does not mean that the Celtic peoples from Britain did not have any cultural notions of the Mother Goddesses, only that some particularities in the worship must have come with the army. The cult of the Mothers in Britain is clearly Romanized, for they all bear Roman epithets, such as Transmarinae, Campestres, Domesticae or Fatae, apart from the Matres Ollototae and the Matres Suleviae. The Matres Ollototae are undeniably Celtic, for their name is composed of Celtic ollo-, ‘all’ and teuta, touta, ‘tribe’. They are thus ‘The Mothers of All the Peoples’. They are mentioned in an inscription from Heronbridge (Claverton, Cheshire): Deabus Matribus Ollototis Iul(ius) Secundus et Aelia Augustina, ‘To the Mother Goddesses Ollototae, Julius Secundus and Aelia Augustina (set this up)’, and in three inscriptions from Binchester (Durham): Deab(us) Matrib(us) O[l]lot(otis) T[i]b(erius) Cl(audius) Quintianus b(ene)f(iciarius) co(n)s(ularis) v.s.l.m., ‘To the Mother Goddesses Ollototae Tiberius Claudius Quintianus beneficiaries of the governor, willingly and deservedly fulfilled his vow’ ; [M]atrib(us) O[lloto(tis)] CARTO VAL MARTI Vetto(num) GENIO LOCI LIT . IXT, ‘To the Mother Goddesses Ollototae … Cavalry Regiment of Vettonians….’ ; I(ovi) O(ptimo) M(axiom) et Matribus Ollototis sive Transmarinis, ‘To Jupiter, Best and Greatest, and to the Ollototae or Overseas Mother Goddesses’.”