During their occupation of Britain the Romans built an extensive network of roads which continued to be used in later centuries and many are still followed today. The Romans also built water supply, sanitation and sewage systems. Many of Britain's major cities, such as London (Londinium), Manchester (Mamucium) and York (Eboracum), were founded by the Romans. Britain is the largest European region of the former Western Roman Empire whose majority language is neither: A Romance language, A language descended from the pre-Roman inhabitants, though Welsh exists as a living minority language, with many borrowings from Latin, such as llaeth ("milk", latte), ffenestr ("window", finestra). Fragmentary use of Cornish lasted into the early modern period and the language has been seeing a renaissance since the late 20th century. Towards the end of the 4th century Britain came under increasing pressure from barbarian attacks, and there were not enough troops to mount an effective defence. After elevating two disappointing usurpers, the army chose a soldier, Constantine III, to become emperor in 407. He crossed to Gaul but was defeated by Honorius; it is unclear how many troops remained or ever returned, or whether a commander-in-chief in Britain was ever reappointed. A Saxonincursion in 408 was apparently repelled by the Britons, and in 409 Zosimus records that the natives expelled the Roman civilian administration. However, Zosimus may be referring to the Bacaudic rebellion of the Breton inhabitants of Armorica since he describes how, in the aftermath of the revolt, all of Armorica and the rest of Gaul followed the example of the Brettaniai. A letter from Emperor Honorius in 410 has traditionally been seen as rejecting a British appeal for help, but it may have been addressed to Bruttium or Bologna. With the imperial layers of the military and civil government gone, administration and justice fell to municipal authorities, and local warlords gradually emerged all over Britain, still utilizing Romano-British ideals and conventions. Laycock has investigated this process and emphasised elements of continuity from the British tribes in the pre-Roman and Roman periods, through to the native post-Roman kingdoms. In British/Welsh tradition, pagan Saxons were invited by Vortigern to assist in fighting the Picts and Irish, though Germanic migration into Roman Britannia may have begun much earlier. There is recorded evidence, for example, of Germanic auxiliaries supporting the legions in Britain in the 1st and 2nd centuries. The new arrivals rebelled, plunging the country into a series of wars that eventually led to the Saxon occupation of Lowland Britain by 600. Around this time many Britons fled to Brittany, Galicia and probably Ireland. A significant date in sub-Roman Britain is the Groans of the Britons, an unanswered appeal to Aetius, leading general of the western Empire, for assistance against Saxon invasion in 446. Another is the Battle of Deorham in 577, after which the significant cities of Bath, Cirencester and Gloucester fell and the Saxons reached the western sea.
Ambrosius Aurelianus, called Aurelius Ambrosius in the Historia Regum Britanniae and elsewhere, was a war leader of the Romano-British who won an important battle against the Anglo-Saxons in the 5th century, according to Gildas. He also appeared independently in the legends of the Britons, beginning with the 9th-century Historia Brittonum. Eventually he was transformed into the uncle of King Arthur, the brother of Arthur's father Uther Pendragon, and predeceases them both. Due to Gildas' description of him, Ambrosius is one of the figures called the Last of the Romans. Ambrosius Aurelianus is one of the few people that Gildas identifies by name in his sermon De Excidio et Conquestu Britanniae, and the only one named from the 5th century. De Excidio is considered the oldest extant British document about the so-called Arthurian period of Sub-Roman Britain. Following the destructive assault of the Saxons, the survivors gather together under the leadership of Ambrosius, who is described as: "... a gentleman who, perhaps alone of the Romans, had survived the shock of this notable storm. Certainly his parents, who had worn the purple, were slain in it. His descendants in our day have become greatly inferior to their grandfather's [avita] excellence. Under him our people regained their strength, and challenged the victors to battle. The Lord assented, and the battle went their way." Some basic information on Ambrosius can be deduced from the brief passage: Ambrosius was possibly of high birth and very likely a Christian (Gildas says that he won his battles "with God 's help"). Ambrosius' parents were slain by the Saxons and he was among the few survivors of their initial invasion. According to Gildas, Ambrosius organised the survivors into an armed force and achieved the first military victory over the Saxon invaders. However, this victory was not decisive: "Sometimes the Saxons and sometimes the citizens [meaning the Romano-British inhabitants] were victorious." Peter Korrel supports the view that Ambrosius Aurelianus and King Arthur might be the same person, a theory also supported by others debating the historicity of King Arthur. He points that all accounts about Ambrosius can be traced back to Gildas and all accounts about Arthur can be traced back to Nennius, with the two figures being very similar. Both figures have been connected to the Battle of Badon and have been portrayed as its sole hero. Their identity as kings is a later addition; they are originally depicted as leaders of the Britons in their wars with the Saxons. The Latin term dux bellorum can be applied to both of them. Both figures are of Roman descent. With Ambrosius, this is featured in both his name and stated background. With Arthur, this is implied, since the name seems to be derived from Artorius, the name of a minor Roman gens. Both figures are known for their valour. Ambrosius is featured as the one who manages to re-assemble the desperate Britons, encourage them to fight once again, and make a stand against the seemingly superior enemy. This would require personal courage. Arthur is reported to have single-handedly faced numerically superior enemies. Both figures are devout Christians whose victories are attributed to divine protection. In both cases, the Battle of Badon seems to be the final victory of a long war. Bedas states that Ambrosius started the war against the Saxons and both factions had victories and losses until the war concluded in the Battle of Badon. In Arthurian narratives, Arthur is said to have led the Britons in 12 victories, with the Battle of Badon being the last of them. Korrel admits that there is a significant difference. Ambrosius both won and lost battles, while Arthur reportedly never lost a battle. But this is probably the difference between a historical figure and one featured in epics.
The Battle of Badon (Bellum in monte Badonis or Mons Badonicus) was a battle thought to have occurred between Britons and Anglo-Saxons in the late 5th or early 6th century. It was credited as a major victory for the Britons, stopping the encroachment of the Anglo-Saxon kingdoms for a period. It is chiefly known today for the supposed involvement of King Arthur, a tradition that first clearly appeared in the 9th-century Historia Brittonum. Because of the limited number of sources, there is no certainty about the date, location, or details of the fighting. The earliest mention of the Battle of Badon is Gildas' De Excidio et Conquestu Britanniae ("On the Ruin and Conquest of Britain"), written in the early to mid- 6th century. In it, the Anglo-Saxons are said to have "dipped [their] red and savage tongue in the western ocean" before Ambrosius Aurelianus organized a British resistance with the survivors of the initial Saxon onslaught. Gildas describes the period that followed Ambrosius' initial success: " From that time, the citizens were sometimes victorious, sometimes the enemy, in order that the Lord, according to His wont, might try in this nation the Israel of to-day, whether it loves Him or not. This continued up to the year of the siege of Badon Hill (obsessionis Badonici montis ), and of almost the last great slaughter inflicted upon the rascally crew. And this commences, a fact I know, as the forty-fourth year, with one month now elapsed; it is also the year of my birth." The Ruin of Britain is unclear as to whether Ambrosius is still leading the Britons at this point, but describes the battle as such an "unexpected recovery of the [island]" that it caused kings, nobles, priests, and commoners to "live orderly according to their several vocations" before the long peace degenerated into civil wars and the iniquity of Maelgwn Gwynedd. Passages of The Ruin of Britain that address Maelgwn directly are sometimes employed to date the work from accounts of the king's death by plague in the 540s, but such arguments ignore the obvious apostrophe employed in the passages and the possible years of composition involved in the final collected sermon. The battle is next mentioned in an 8th century text of Bede's Ecclesiastical History of the English People. It describes the "siege of Mount Badon, when they made no small slaughter of those invaders," as occurring 44 years after the first Anglo-Saxon settlement of Britain. Since Bede places that arrival during or just after the joint reign of Marcian and Valentinian in AD 449–456, he must have considered Badon to have taken place between 493 and 500. Bede then puts off discussion of the battle, "But more of this hereafter", only to seemingly never return to it. Bede does later include an extended account of Saint Germanus 's victory over the Saxons and Picts in a mountain valley, which he credits with curbing the threat of invasion for a generation. However, as the victory is described as having been accomplished bloodlessly, it was presumably a different occasion from Badon. (Accepted at face value, St. Germanus's involvement would also place the battle around 430, although Bede's chronology shows no knowledge of this.) The earliest surviving text mentioning Arthur at the battle is the early 9th century Historia Brittonum, in which the soldier (Latin mīles) Arthur is identified as the leader of the victorious British force at Badon: "The twelfth battle was on Mount Badon in which there fell in one day 960 men from one charge by Arthur; and no one struck them down except Arthur himself". The Battle of Badon is next mentioned in the Annales Cambriae ("Annals of Wales"), assumed to have been written during the mid- to late-10th century. The entry states: The Battle of Badon, in which Arthur carried the Cross of our Lord Jesus Christ for three days and three nights upon his shoulders (or shield) and the Britons were the victors". That Arthur had gone unmentioned in the source closest to his own time, Gildas, was noticed at least as early as the 12th century hagiography that claims that Gildas had praised Arthur extensively but then excised him completely after Arthur killed the saint's brother, Hueil mab Caw. Modern writers have suggested the details of the battle were so well known that Gildas could have expected his audience to be familiar with them. Geoffrey of Monmouth's c. 1136 Historia Regum Britanniae was massively popular and survives in many copies from soon after its composition. Going into (and fabricating) much greater detail, Geoffrey closely identifies Badon with Bath, including having Merlin foretell that Badon's baths would lose their hot water and turn poisonous. He employs aspects of other accounts, mixing them: the battle begins as a Saxon siege and then becomes a normal engagement once Arthur's men arrive; Arthur bears the image of the Virgin both on his shield and shoulder. Arthur charges, but kills a mere 470, ten more than the number of Britons ambushed by Hengist near Salisbury. Elements of the Welsh legends are also added: in addition to the shield Pridwen, Arthur gains his sword Caliburnus and his spear, Ron. Geoffrey also makes the defence of the city from the Saxon sneak attack a holy cause, having Dubricius offer absolution of all sins for those who fall in battle. The early sources' account that the Saxons were thrown back around this time seems to be borne out by archaeological evidence. Studies of cemeteries (at this point, the Anglo-Saxons remained pagan while the Britons were Christianized) suggest the border shifted some time around 500. Afterwards, the pagans held the present areas of Kent , Sussex, Norfolk and Suffolk, and the area around the Humber. The Britons seem to have controlled salients to the north and west of London and south of Verulamium in addition to everything west of a line running from Christchurch at the mouth of the Wiltshire Avon north to the Trent, then along the Trent to the Humber, then north along the Derwent to the North Sea. The salients could then be supplied along Watling Street, dividing the invaders into pockets south of the Weald in east Kent and around the Wash.