Ward-Perkins suggests the Welsh had to abandon those Roman ways which proved insufficient, or indeed superfluous, to meet the challenge of survival they faced, "Militarized tribal societies, despite their political fragmentation and internecine strife, seem to have offered better protection against Germanic invasion than exclusive dependence on a professional Roman army (that in the troubled years of the fifth century was all too prone to melt away or mutiny)." The region of Venedotia, however, had been under Roman military administration and included established Irish Gaelic settlements, and the civilian element there was less extensive, perhaps facilitating technological loss.
In the post-Roman period, the earliest rulers of Wales and Gwynedd may have exerted authority over regions no larger than the cantrefi (hundred) described in Welsh law codified centuries later, with their size somewhat comparable in size to the Irish tuath.
The Battle of Chester would not end the ability of the Welsh to seriously threaten the Anglo-Saxon polities.
Among the most powerful of the early kings of Gwynedd was Cadwallon ap Cadfan (c. He became engaged in an initially disastrous campaign against Northumbria where following a series of epic defeats he was confined first to Anglesey and then just to Puffin Island before being forced into exile across the Irish Sea to Dublin – a place which would come to host many royal refugees from Gwynedd.
Rhun returned to Gwynedd and the rest of his reign was far less eventful. In a rare show of common interest, it appears Gwynedd and neighbouring Kingdom of Powys acted in concert to rebuff the Anglican advance but were defeated at the Battle of Chester in 613.
Following this catastrophe the approximate borders of northern Wales were set with the city of Caerlleon (now called Chester) and the surrounding Cheshire Plain falling under the control of the Anglo-Saxons.
are said to have arrived in Anglesey and elsewhere in Northwest Wales, with the name Llŷn derived from Laigin, an Old Irish form that means "of Leinster". The name was initially attributed to a specific Irish colony on Anglesey, but broadened to refer to Irish settlers as a whole in North Wales by the 5th century.
According to 9th century monk and chronicler, Nennius, North Wales was left defenseless by the Roman withdrawal and subject to increasing raids by marauders from Man and Ireland, a situation which led Cunedda, his sons and their entourage, to migrate in the mid-5th century from Manaw Gododdin (now Clackmannanshire, Scotland) to settle and defend North Wales against the raiders and bring the region within Romano-British control.
It is in memory of a man named Cantiorix and the Latin inscription is Cantiorix hic iacit/Venedotis cives fuit/consobrinos Magli magistrati: "Cantiorix lies here.
He was a citizen of Gwynedd and a cousin of Maglos the magistrate".
All must have seemed lost but Cadwallon raised an enormous army and after a brief time in Guernsey he invaded Dumnonia, relieved the West Welsh who were suffering a Mercian invasion and forced the pagan Penda of Mercia into an alliance against Northumbria.
With new vigour Cadwallon returned to his Northumbrian foes, devastated their armies and slaughtered a series of their kings.
Other evidence supports Nennius's claim that a leader came to north Wales and brought the region a measure of stability, although an Irish Gaelic element remained until the mid-5th century.