Belinus, for his part, returned to Britain and for the remaining days of his life governed his homeland in peace. He restored existing cities wherever they had fallen into decay and he founded many new ones. Among the others which he founded was a certain city on the bank of the River Usk, near to the Severn Sea: this was the capital of Demetia and for a long time it was called Kaerusc. When the Romans came the earlier name was dropped and it was re-named the City of the Legions, taking its title from the Roman legions who used to winter there. [III.10]

Gurguit Barbtruc’s death was a peaceful one. He was buried in the City of the Legions which, ever since his father’s death, he had done so much to adorn with walls and public buildings. [III.12]

CAERLEON, or as Geoffrey most often calls it, the City of the Legions, appears first in the Historia when founded by Belinus and expanded by Belinus’ son. Geoffrey explains the name with reference to the Romans. Caerleon does indeed have significant Roman remains, though Tatlock notes that it was a military outpost rather than a town (p. 70): the Roman history is traced on Caerleon.net.

Most striking is Geoffrey’s decision to make the city King Arthur's capital. Arthur holds his great plenary court at Caerleon, and the lengthy description of the activities of that court begins with a detailed description of the city itself:

Situated as it is in Glamorganshire, on the River Usk, not far from the Severn Sea, in a most pleasant position, and being richer in material wealth than other townships, this city was eminently suitable for such a ceremony. The river which I have named flowed by it on one side, and up this the kings and princes who were to come from across the sea could be carried in a fleet of ships. On the other side, which was flanked by meadows and wooded groves, they had adorned the city with royal palaces, and by the gold-painted gables of its roofs it was a match for Rome. What is more, it was famous for its two churches. One of these, built in honour of the martyr Julius, was graced by a choir of most lovely virgins dedicated to God. The second, founded in the name of the blessed Aaron, the companion of Julius, was served by a monastery of canons, and counted as the third metropolitan see of Britain. The city also contained a college of two hundred learned men, who were skilled in astronomy and the other arts, and who watched with great attention the courses of the stars and so by their careful computations prophesied for King Arthur any prodigies due at that time. [IX.12]

Tatlock notes that all of the references to the site and the buildings are more or less accurate (p. 70). The religious prominence of the city is more of a surprise. Geoffrey consistently presents the City of the Legions as one of the chief metropolitan sees:

At that time there were twenty-eight flamens in Britain and three archflamens.... At the Pope’s bidding, the missionaries converted these men from their idolatry. Where there were flamens they placed bishops and where there were archflamens they appointed archbishops. The seats of the archflamens had been in three noble cities, London, York, and the City of the Legions, the site of which last, by the River Usk in Glamorgan, is still shown by its ancient walls and buildings.... The Severn divides these last two provinces from Kambria or Wales, which last was placed under the City of the Legions. [IV.19]

Tremorinus is the bishop of the City in the time of Aurelius,VIII.10, who appoints Dubricius as successor after Tremorinus’ death [VIII.12]. Dubricius is later described as the Primate of Britain and the legate of the Papal See [IX.12], though he eventually resigns to lead a hermit’s life [IX.15].

It was then, too, that David, most holy Archbishop of the City of the Legions, died in the town of Menevia, inside his own abbey, which he loved more than all the other monasteries of his diocese, for St. Patrick, who prophesied David’s own birth, had founded it.... Kinoc, the priest of the church of Llanbadarn, was promoted to this higher rank and replaced him in the metropolitan see. [XI.3]

 

There do seem to have been churches for the martyrs Julius and Aaron, whose deaths feature briefly in the Historia:

Among the people of either sex who, with the greatest possible courage, stood firm in the battle-line of Christ, were Albanus, who suffered at St Albans, and Julius and Aaron, two townsfolk of the City of the Legions. [V.5]

But the prominence of Caerleon as an ecclesiastical centre seems to be original to Geoffrey. Tatlock suggests that Geoffrey was motivated by contemporary squabbles over territory and supremacy between LLANDAFF and ST. DAVID’S (p. 71).

Caerleon appears once in Merlin’s prophecies:

Menevia shall be dressed in the pall of the City of the Legions, and a preacher from Ireland shall be struck dumb by a child still growing in the womb. [VII.3]

And it is the site to which Guinevere flees upon news of Arthur’s return to Britain:

The Perjurer reformed his army and so marched into Winchester on the following night. When this was announced to Queen Guinevere, she gave way to despair. She fled from York to the City of the Legions and there, in the church of Julius the Martyr, she took her vows among the nuns, promising to lead a chaste life. [XI.1]