That charming place was laid out with groves, abounding in animals, rich in treasures, pleasant for its green meadows, and watered by the Usk River...Here was the metropolitan city of the province of Demetia; here the legions of Rome used to spend the winters; here King Arthur celebrated the high feasts, wore the crown, and convened all the barons of Britannia for his assembly.
From the rise of Gawain, Nephew of Arthur by Mildred Leake Day.
Caerleon is a small Welsh town beside the River Usk. It's name is Welsh for "City" or Fort of the Legion." Geoffrey of Monmouth calls it the City of Legions, detailing the lavish court that Arthur holds there foreshadowing the Camelot to come.
For it was located in a delightful spot in Glamorgan, on the River Usk, not far from the Severn Sea. Abounding in wealth more than other cities, it was suited for such a ceremony. For the noble river I have named flows along it on one side, upon which the kings and princes who would be coming from overseas could be carried by ship. But on the other side, protected by meadow and woods, it was remarkable for royal palaces, so that it imitated Rome in the golden roofs of its buildings ...Famous for so may pleasant features, Caerleon was made ready for the announced feast.
The History of the Kings of Britain, Geoffrey of Monmouth
In later legend, Camelot doesn't replace Caerleon entirely; it continues to have a presence in the Arthuian legend as a minor court. It was also claimed as the scene of Arthur's ninth battle in Nennius' Historia Brittonum.
Before the excavation of the amphitheater in Caerleon it was buried under a mound of soil with an oval shape. This was claimed by the locals to be the "real" Round Table. Local legend tells of an undergound chamber in the woods where a thousand of Arthur's soldiers lie sleeping, awaiting the day when Wales will need them.