According to Geoffrey of Monmouth, Caerleon, a word derived from the Welsh ‘fortress of the legion’, was the seat from which King Arthur ruled Britain.   He wrote:  http://www.caerleon.net/history/arthur/page7.htm

“When the feast of Whitsuntide began to draw near, Arthur, who was quite overjoyed by his great success, made up his mind to hold a plenary court at that season and place the crown of the kingdom on his head. He decided too, to summon to this feast the leaders who owed him homage, so that he could celebrate Whitsun with greater reverence and renew the closest pacts of peace with his chieftains. He explained to the members of his court what he was proposing to do and accepted their advice that he should carry out his plan in The City Of The Legions.

Situated as it is in Morgannwg (Glamorgan), 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.”

“The temptation is to say that Geoffrey of Monmouth placed King Arthur close to his own home is great, especially since subsequent excavations have unearthed nothing which suggests use of the kind Geoffrey of Monmouth described at any time after the Roman withdrawal.”   http://www.arthurianadventure.com/caerleon-on-usk.htm

The Romans began building a legionnaire fortress in Gwent at Caerleon, Isca Silurum, in 75 AD.  “They built a large “playing-card” shaped fort with initially a timber palisade which was later replaced in stone. The interior was fitted out with the usual array of military buildings: a headquarters building, legate’s residence, tribunes’ houses, hospital, large bath house, workshops, barrack blocks, granaries and an amphitheatre.”  http://en.wikipedia.org/wiki/Isca_Augusta

It was the home of the Legio secunda Augusta (Second Augustan Legion), one of thirty legions that occupied Britain.  It was one of four legions that were part of the initial invasion of Britain (24,000 men in all), failed to answer the call for men to take part in the defeat Boudicca (and was disgraced), and helped build Hadrian’s wall.  http://en.wikipedia.org/wiki/Legio_II_Augusta   http://en.wikipedia.org/wiki/Boudica

Around 290 AD, the legion abandoned Isca. Before they left, they demolished the main buildings.  http://www.caerleon.net/history/army/page6.html

“The Roman name of the Caerleon fortress occurs in three separate itinera within the late-second century list of imperial road-routes known as the Antonine Itinerary, serving as a terminus on two of these routes.

• Iter XII: “the route from Muridunum to Viroconium“, Caerleon appears as Iscae the home of Legio Secunda Augusta, some twenty-seven miles from Bomium (nr. Bridgend, Mid Glamorgan) and nine miles from Burrium (Usk, Gwent).
• Iter XIII: “the route from Isca to Calleva, one-hundred and nine thousand paces”; a road journey which again starts off with the nine mile road to Burrium (Usk, Gwent), then via Blestium (Monmouth, Gwent) a further eleven miles further along the north bank of the Severn Estuary.
• Iter XIV: “an alternative route from Isca to Calleva, one-hundred and three thousand paces”, begins with a nine mile road trip to the civitas capital Venta Silurum (Caerwent, Gwent), followed by a fourteen mile journey across the Bristol Channel to Abona (Sea Mills, Avon).

The only other classical geography which mentions the Caerleon fort is the Ravenna Cosmology of the seventh century, where the name Isca Augusta (R&C#52) occurs between the entry for Bannio (Abergavenny, Gwent) and the unidentified station Albinumno.”  http://www.roman-britain.org/places/isca_silurum.htm