The history of Cornwall begins with the pre-Roman inhabitants, including speakers of a Celtic language that would develop into Brythonic and Cornish. During the time of Roman dominance in Britain, Cornwall was rather remote from the main centres of Romanization. Major Roman roads extended no further west than Isca Dumnoniorum (Exeter). Furthermore, the British tin trade had been largely eclipsed by the more convenient supply from Iberia. After a period of Roman rule, Cornwall reverted to independent Celtic chieftains. In the wake of the Roman withdrawal from Britain, Saxons and other peoples were able to conquer most of the east of the island. Cornwall, however, remained under the rule of local Romano-British and Celtic élites. It appears that Cornwall was either a division of the Dumnonii tribe - whose tribal centre was in the modern county of Devon - or they were a separate tribal entity subordinate to them. During the sub-Roman historic period there is no distinction made between Cornwall and Dumnonia. There may have been a tribal group called the Cornobi on account of a reference to a place within the lands of the Dumnonii by Ptolemy called DVROCORNAVIVM which would imply there was a group of people called the Cornavi or "people of the horn".
The Cornish saints were often closely connected to the local civil rulers; in a number of cases, the saints were also kings. A Kingdom of Cornwall emerged around the 6th century; its kings were at first sub-kings and then successors of the Brythonic Celtic Kingdom of Dumnonia. The political situation was much in flux, and several kings or polities appear to have exercised sovereignty across the Channel in Brittany. Meanwhile the Saxons of Wessex were rapidly approaching from the east and crushing the kingdom of Dumnonia. In 710 it is recorded that the "West Welsh" defeated the men of Wessex but were soon to lose more territory in the years that followed it. In 825 at the Battle of Galford the "Cornish men" in alliance with the Danes attacked Wessex and seem to have kept the status quo and retained political independence for the British lands west of the River Tamar. In another battle c.875 it is recorded that a king Doniert or Dungarth drowns and from this time onwards Cornwall is subject to England. A remarkable quote is attributed to the last independent king of Cornwall; "Sorrow comes from a world upturned". In 936, Athelstan is recorded as having fixed the eastern boundary of the Britons at the Tamar, massacring many of those remaining to the east.
This is also the period known as the 'age of the saints', as Celtic Christianity and a revival of Celtic art spread from Ireland and Scotland into Great Britain, Brittany, and beyond. Cornish saints such as Piran, Meriasek, or Geraint exercised a religious and arguably political influence; their activities also connected Cornwall strongly with Ireland, Brittany, Scotland, and Wales, where many of these saints were trained or formed monasteries. For most of the history, at least until the mid 8th Century, the rulers of Dumnonia were also the rulers of Cornwall. In Arthurian legend Gorlois (Gwrlais in Welsh) is attributed the title "Duke of Cornwall" but evidence of his existence is scant. He could have been a sub-king in Cornwall because of place names such as Carhurles (Caer-Wrlais) and Treworlas (Tre-Wrlais). It is also possible that he may have ruled the Cornobi who were a tribe with a similar name living in the Gloucestershire area. There was certainly a King Mark of Cornwall and a King Tristan. After the loss of the territory today called Devon the British rulers are referred to either as the kings of Cornwall or the kings of the "West Welsh".
Petroc and sixty of his retainers set sail for Ireland where they visited "as a native rather than as a stranger all the famous seats of study and religion". Their wanderings and instruction in monastic ways is described in the "Vita Petroci" as lasting twenty years. St. Petroc is recorded as having arrived at the mouth of the river Camel, near Trebetherick. Trebetherick is but a stone's throw from Padstow and it was to this ancient seaport that St. Petroc and his monks came around 600 A.D. There, St. Petroc and his followers established themselves in the Celtic Monastery of Lanwethinoc, which was founded by the Bishop Wethinoc. The monastery became known as Petrocstow, Petroc's Church. It is interesting to note that the name Lanwethnoc remained long enough to be recorded in the Domesday Book and referred to the Manor of Padstow.