When the Romans left Britain, they didn’t just take their belongings with them; they took their entire way of life. Over the centuries, Roman rule had civilized Britain. The abrupt departure of the Romans left a vacuum the Anglo-Saxons were happy to fill, but the two invaders could not have been more different. The Romans had introduced government to Britain, central political and economic structures that had created an orderly and prosperous life for most. They had also established long-distance trade, money, taxes, roads, sanitation, pottery, and glass. When the Romans left, they took all of these innovations with them and under the illiterate, “barbaric” Anglo-Saxons the British were reduced to a barter economy and lived in a more primitive state than their ancestors before.

Those who were able to escape fled to Armorica in Gaul (modern-day France) where they settled by the sea in a land that closely resembled that which they had left behind. To this day, inhabitants of the part of France now known as Brittany speak in an unusual Welsh-sounding dialect that is an ancient British tongue. For those who remained behind in Britain, the only option was to run and hide from the fearful barbaric warriors who would kill them on sight.

It is at this desperate juncture in British history that Ambrosius Aurelianus appears and the legend of King Arthur is born. Little can be fact-checked on the life of Ambrosius Aurelianus, but it is thought that he was a Roman general of impeccable lineage who had remained behind in Britain when the Empire fell. Aurelianus may have had a son who was Romano-Celtic and given the same name, and as a result, the British army of resistance against the Anglo-Saxons may have been active under a leader named Aurelianus for two generations.

Under the second Aurelianus, who has been described as a Welsh prince, the first effective resistance to Anglo-Saxon forces was established. Saxons were pushing further and further west, forcing Romano-British deeper into Wales. Aurelianus gathered the surviving Roman-British and organized them into a military force, capable of standing up to the Anglo-Saxons. In a series of battles, the British resistance managed to reclaim important territory, forcing West Saxons out of Dorset and as far as Wiltshire.

These skirmishes reached their climax in the Battle of Mons Badonicus, or the Battle of Mount Badon, which is thought to have taken place around the year 500 AD. We cannot be sure exactly where Mount Badon is. Most of the assumptions historians are able to make about this period of ancient history come from archeological findings. Evidence of a hillfort manned by Romano-British people was found at Little Solsbury Hill in southern England, close to Bath, leading some historians to believe that this is Mount Badon. Others think it more likely that Aurelianus was defending the north of England and that Mount Badon is probably located in Cumbria, known as Camboglanna in Roman times.

What little information we have about Ambrosius Aurelianus and the Battle of Mount Badon comes from just one scant source: the sermon of a sixth-century British priest or monk named Gildas. Gildas, who was known as Gildas the Wise, wrote the sermon De Excidio et Conquestu Britanniae (On the Ruin and Conquest of Britain) as a religious re-telling of the Anglo-Saxon invasion of Britain and the aftermath. It is thought that Gildas’ text was written sometime between 510 and 540 AD, meaning Gildas was writing about events as they were happening or at the very least had happened within living memory. As such, Gildas’ text is incredibly valuable to historians studying this particular period of British history. Gildas, for example, offers one of the very first descriptions of Hadrian’s Wall.

On Ambrosius Aurelianus, Gildas says only, “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 excellence. Under him, our people regained their strength and challenged the victors to battle. The Lord assented, and the battle went their way.”

Gildas account of Aurelianus and his followers’ Christian piety and bravery in this first section of his sermon contrasts with the next two parts in which he condemns contemporary leaders for their sinful ways. Was Aurelianus a real man? And if so, was his life the foundation of the legend of King Arthur? One theory suggests that Arthur was a nickname given to Aurelianus by his men. In early Anglo-Saxon times, Arthur meant “bear man” and alluded either to Aurelianus being a particularly powerful and hairy man or to his habit of wearing a bear skin cloak.

The next mention of the British leader who led his men to victory at Mount Badon comes from the Welsh cleric Nennius, who wrote Historia Brittonum (History of Britain) around 830 AD. Nennius lists twelve battles between the British and the Anglo-Saxons, but unlike Gildas, he names the leader of the battles Arthur. Arthur is briefly mentioned again in the slightly later Annales Cambriae, compiled during the seventh or eighth century. This weighty chronicle was written anonymously and includes references to the Battle of Badon during “year 72” with the detail, “Arthur bore the Cross of our Lord Jesus Christ on his shoulders for three days and three nights and the Britons were the victors.” Arthur’s death is also listed as having occurred during a battle at Camlann in 539 AD. It’s not much to go on, but it was enough to spark the imagination of subsequent generations who looked to these ancient texts as proof of the origin of the legend of King Arthur.

The Battle of Mount Badon became legendary as it was the first military victory the Romano-British had achieved over the Anglo-Saxons and secured relative peace for 50 years. Peace was disrupted again in 550 AD when a massive and devastating new wave of Saxons descended on Britain and took almost complete control of the land. By the seventh century, there was no such thing as Britain; four distinct cultures were sharing the cluster of islands we now refer to as the United Kingdom.

What remained of the Romano-British stayed in Wales and the southwest, but by now they had taken on the language of the Celts. A culture known as the Gaels lived in Ireland and the northwestern region of Scotland. The Pictish kingdoms kept control of the land north of the Roman Wall. And the Anglo-Saxons and Jutes controlled the vast majority of England, territories they named Mercia, Northumbria, East Anglia, Kent, Wessex, Sussex, and Essex. The very term Welsh is an Anglo-Saxon one, derived from Wielisc or Wyliscand meaning foreign or slave. Even today, the Welsh term used to describe England translates roughly as “the lost lands.”

It’s impossible to say how much of Aurelianus or King Arthur’s life happened, and focusing too much on historical evidence is beside the point. The time period in which King Arthur was supposed to have lived was a real period in British history that later became known as the Dark Ages. Looking back at the Anglo-Saxon invasion their ancestors lived through, subsequent generations needed a hero whose bravery and Christian virtues were something to look up to, and this brings us nicely to the legend of King Arthur in the twelfth century.