Following the end of Roman rule, power devolved on the local Romano-British elites who were responsible for local government under Roman rule. Many aristocratic Britons initially retained the titles of their Roman magistracies - St Patrick’s father was a decurion (a town councillor) and in 429 Verulamium (St Albans) was ruled by a man claiming the power of a tribune - and it is likely that they tried to preserve a Roman-style administration. After all, this was the only example of government they would have known. In the longer term, political and economic dislocation made this both unsustainable and unnecessary and local aristocrats set themselves up as kings and tyranni {'tyrants'). In some cases these kingdoms were based on a Roman city; Gloucester, Cirencester and Bath all had their own kings in the sixth century. In the less Romanised west and north, it was the old Iron Age tribal identities that formed the basis of emerging kingdoms. The Cumbrian kingdom of Rheged was formed by the Novantae or Carvetii, the north Welsh kingdom of Gwynedd by the Ordovices, and in Devon and Cornwall the kingdom of Dumnonia by the Dumnonii. It might be expected that efforts to maintain a Romanised administration would have been strongest in the wealthy south-east, but this was the first area to be overrun by the Anglo-Saxons, so little is known about political structures there. It is not clear either what efforts were made to preserve a degree of unity in the face of the external threats faced by the Britons, but there were some rulers such as the ‘great tyrant’ Vortigern (‘overlord’) and Ambrosius Aurelianus whose leadership was widely recognised by the Britons. It was, in fact, the clergy, rather than secular rulers, who preserved the most important aspects of Roman influence on the Britons - the Latin language and the Christian religion - ensuring that no matter how politically fragmented it became, Britain remained culturally a part of the late Roman world after 410. The so-called Celtic church developed only in the later fifth century, after the pagan Anglo-Saxons overran south-eastern Britain. Communication between the British church and Rome became more difficult, allowing doctrinal differences, notably in calculating the date of Easter, to develop.
The most objective picture of what was actually happening in Britain in the century following the end of Roman rule comes from archaeology. Pervasive insecurity and economic dislocation had already led to a decline in urban life and the abandonment of most country villas by the end of the fifth century. Nevertheless, for a time at least, the Britons attempted to maintain the infrastructure of a Romanised lifestyle. For example, the bath complex at Bath was maintained until as late as c. 470. At Verulamium the forum remained in use, and water mains and new mosaic floors were being laid in high-status houses until as late as c. 475. Wroxeter in Shropshire bucked the trend completely and saw a major redevelopment in the middle of the fifth century, when new shops and a large timber mansion - possibly the residence of a local king - were built. Carlisle also saw new buildings in the early fifth century, including a large timber hall, which may have been an assembly place for the local kingdom of Rheged. There may be other such halls awaiting discovery in other Roman cities, which retained a symbolic importance as centres of power long after they had ceased to function as true urban centres. The headquarters building of the legionary fortress at York was maintained for centuries, probably as the headquarters of the British, and later Anglian, kings of Deira. A few Roman forts, such as Birdoswald on Hadrian’s Wall, developed into villages, occupied no doubt by descendants of their original garrisons, but, perhaps surprisingly in view of their well-built stone walls, they were not attractive places for the Britons to settle.
Though urban life of some sort seems to have continued into the early sixth century at Carlisle and Wroxeter, by this time many of the towns of Roman Britain had been abandoned, supplanted by hillforts. Some of these, such as South Cadbury in Somerset, were reoccupied Iron Age hill-forts but most were new foundations. These post-Roman hillforts were much smaller than their Iron Age antecedents and were clearly intended only to accommodate a local chieftain or king and his elite warriors and servants, rather than an entire tribal population. Their builders exhibited a marked preference for natural strongholds - small, steep rocky hills - such as Dinas Emrys in the heart of Snowdonia and the limestone crag of Dinas Powys near Cardiff. In this way the former Roman province came to resemble the un-Romanised area north of the wall, or even Ireland.
These power centres may have been cramped and exposed but they were certainly not isolated. Fortified power centres from Castle Dore in Cornwall to Dunadd in Argyll, well north of the Romanised area, have produced evidence of trade contacts reaching as far afield as the eastern Mediterranean in the shape of wine amphorae and other goods. The spread of Christianity made obtaining wine important for religious as well as social reasons because of its use in celebrating the Eucharist (or ‘Mass’). Early monastic sites, such as that at Tintagel in Cornwall, shared the same wide-ranging trade connections as the elite power centres. Now that the old Roman port towns had been abandoned, goods were landed at informal beach sites on sheltered estuaries like a recently excavated site among sand dunes at Bantham in south Devon that produced huge amounts of pottery from the Aegean, Anatolia, Syria and North Africa. Negotiations held here between the merchants and the local elite were treated as major social events, accompanied by feasting on cattle, sheep, pigs, deer and other mammals and wildfowl, which were roasted over huge outdoor hearths.
It is clear that the newly independent Britons believed the greatest threat to their way of life came from their un-Romanised and still pagan fellow Celts, the Piets and the Scots/Irish, rather than the Saxons. The Piets were fierce raiders but the Scots were the more dangerous because they came as conquerors and settlers too. Historical traditions and memorial stones inscribed with the Irish ogham alphabet provide evidence of Irish settlements in Devon and Cornwall, south-west Wales, north Wales, the Isle of Man and Argyll. In the long term, the settlement in Argyll was to be the most significant, but that was mainly a problem for the Piets. The tyrant Vortigern is said to have reacted to the threat of the Piets and Scots by ordering Cunedda, a prince of the Gododdin (i. e. the Votadini), to North Wales to drive out the Irish invaders, which he duly did. Cunedda succeeded in establishing himself as ruler of the Ordovices and it was from him that the kings of Gwynedd claimed descent. Later historical traditions of Gwynedd claimed that Cunedda also drove out the Irish from southwest Wales, but this was a fiction intended to bolster the dynasty’s claims to overlordship of all Wales. Medieval Welsh genealogies include Irish names among the early kings of Dyfed (in south-west Wales), showing that the Irish here established a successful dynasty. However, these Irish settlers were soon assimilated with the native Britons, as ogham and Old Irish was soon superseded on memorial stones by the Roman alphabet and the Latin language. The Irish settlers in Cornwall and Devon were also assimilated with the natives, but the Isle of Man became completely Gaelicised. At least in part, the Britons’ success against the Piets and Scots may have been due to the support of Saxon mercenaries who were invited to settle in Britain in return for military service. The Saxons proved to be dangerous allies.