Nennius - Historia Brittonum
It is thought that Nennius was a Welsh monk who lived some time during the seventh, eighth or ninth centuries.
He wrote a History of Britain, Historia Brittonum, using a range of sources. The result was a collection of documents witten in a range of styles. During later times this work was transcribed with changes and additions made.
Much of 'Historia Brittonum' can be dismissed as little more than fiction, however it would be unwise to totally ignore such an early written source. It is thought that he had access to fifth century works no longer available. It is likely he drew on Welsh folklore - which was not formally recorded in text for many centuries to come.
Historia Brittonum records Caerleon as one of Britains thirty three cities (nearby Caerwent is also listed). It also contains the first written reference to Arthur, described as a warrior who united the Britons in their fight against the Saxons.