General Notes

AMBROSIUS AURELIANUS was, according to the later legends, the High King of the Britons after Vortigern. Although the reality of his role is clouded by much uncertainty, his impact on this period in Britain's history was significant. He is mentioned in four early texts, the first and oldest of which is De Excidio et Conquestu Britanniae, a 6th century diatribe against the lazy and apathetic British people and five corrupt British kings, called "contemptible principalities". It was written by the British monk, Gildas, who says that Ambrosius, alone, is worthy of praise among his countrymen for his leadership of the British counteroffensive against the invading Anglo-Saxons. He is credited with standing against the tide of invasion and heartening his countrymen by his own courage, and he seems to have done much to prevent the early break-up of Celtic Britain in the face of an overwhelming Teutonic onslaught. Gildas refers to him as a "Roman", which clearly implies his continuation of Roman methods of organisation and operation. It also reflects on his family and background, as he seems to have been brought up in a very typically upper class Romanised British environment. Gildas goes on to say that the Saxon advance was halted by a stunning British victory at Mons Badonicus (Mount Badon), which is believed to have been fought around the year 496. This victory so stunned the Teutons (probably under the overall leadership of Aelle of the South Saxons at this time), that an entire generation of peace was bought for the British. This is borne out in archaeological evidence which finds a sudden cessation of Saxon advances in the south until the mid-sixth century. The Anglo-Saxon Chronicle also goes very goes at this stage. Gildas stops short of naming the commander of the home forces at Badon, but subsequent centuries have given that credit to Arthur. This is entirely possible, as Ambrosius was at the height of his powers in the third quarter of the fifth century, and by 496, a replacement commander is more than likely, with Arthur fitting the bill as the then battle leader of the Britons, and perhaps High King.
Our second reference to Ambrosius comes from The Venerable Bede, an eighth century monk of the monastery of Jarrow, in the well written A History of the English Church and People. In a statement which seems to support Gildas, Bede calls him "Ambrosius Aurelius, a modest man of Roman origin, who was the sole survivor of the catastrophe in which his royal parents had perished." That refers to the Saxon foederati revolt which occurred in the early 440s, and spread terror throughout southern Britain, persuading much of the aristocracy to emigrate to the more stable Armorican kingdoms. Bede tells us that "under his leadership the Britons took up arms, challenged their conquerors to battle, and with God's help inflicted a defeat upon them."
Nennius, a monk living in Bangor, was the early 9th century supposed compiler of an eclectic mass of material called the Historia Brittonum. This is a fascinating document of uncertain historical reliability, and was the first serious attempt after Bede to put down the history of the Britons onto paper. Nennius seems to write about two different Ambrosius'. In the first case, he refers to a clearly legendary Ambrosius as being a fatherless child who displayed prophetic powers before Vortigern (he could well have got his references confused here as this refers to the life of Myrvin, the later Merlin of Geoffrey of Monmouth's History of the Kings of Britain). Nennius also says that Ambrosius was a rival whom Vortigern dreaded, and, in a later passage, calls him "the great king of all the kings of the British nation," essentially confirming his status as High King.
What can also be strongly inferred, both from archaeological and textual evidence, is that Ambrosius came to control the West County area around Gloucester. Nennius has Vortigern making a gift of the land, perhaps to try and placate a potential political enemy, but whether Ambrosius gained the land (called Guenet) as a gift, or after the death of Vortigern, he apparently did use the region as a base. If this is true, then in all liklihood he passed it onto his son as a single kingdom which only later split up into the Caer Baddan, Caer Ceri and Caer Gloui that fell to the West Saxons in 577.
Geoffrey of Monmouth calls him Aurelius Ambrosius, and says that when King Constans was murdered by the usurper, Vortigern, Ambrosius and his brother, Uther, were smuggled to Brittany (Armorica) to gain strength to return to campaign against Vortigern. In time, Ambrosius defeated Vortigern, warred successfully against the Saxons and had their leader, Hengist King of the Cantware in Kent, killed (in AD 488, an act which must be falsely attributed to him, as it occured about eight years after his probable date of death). According to Geoffrey, Vortigern's son, Paschent, King of Builth, eventually had Ambrosius poisoned (which fits in much more acceptable with Ambrosius' death earlier in the century, in or around 480, which means Arthur was much more likely to be responsible for Hengist's death and Mount Badon).
British Chronology
Whether Ambrosius was a king of the Britons, a war leader against the Saxons, a Briton, a Roman, all of the above or none of the above, isn't known for sure outside the legends and tales about him. Some have thought that Ambrosius and Arthur are really one and the same, others that he was Arthur's uncle. The truth is probably that Ambrosius Aurelianus was a genuine, heroic, 5th century, Romano-British war leader, some of whose own exploits have been applied to the legend of Arthur. Given the confusion by some over their respective periods of rule, this isn't surprising, but in all likelihood, Ambrosius Aurelianus was at his most active during the civil war against Vortigern, and from the date of the latter's death, circa 455 to somewhere around 480, by which time, it seems likely that Arthur led the battle from around 480 - 511.

SOURCE: http://www.britannia.com/history/biographies/ambros.html
Ambrosius Aurelianus, the second son of the Emperor Constantine, was known to the Welsh as Emrys Wledig (the Imperator) or Emrys Benaur (the Golden-Headed). Geoffrey of Monmouth tells us how he was still a young child when his teenage brother, Constans' short-lived reign came to an abrupt end. With his father executed and his brother murdered, little Ambrosius, along with his brother, Uther, was bundled up and taken across the Channel to the safety of the court of his cousin, Budic I of Brittany. Here he grew up, while the evil Vortigern reigned in Britain, but always Ambrosius planned to return and claim his rightful inheritance.

His chance arrived some years later. Ambrosius returned to Britain, landed at Totnes (Devon) and it may be at this point in history that he clashed with Vitalinus (probably Vortigern or a supporter) at the Battle of Guoloph (Nether Wallop in Hampshire) as recorded by Nennius. This may have resulted in Victory for Ambrosius who was, at some point in history, "given all the kingdoms of the western side of Britain" by Vortigern. Ambrosius was, however, unsatisfied with such a compromise and the struggle between the two continued for most of his life. Vortigern's pro-Saxon policies eventually led to his downfall though and, (probably) in the late 450s, the British people finally rallied behind Ambrosius. Vortigern was hounded into taking refuge in his mountain strongholds. While under siege at Caer-Guorthigirn (Little Doward, Herefordshire), the fortress was miraculously struck by lightning. Vortigern and his entire garrison were burnt to death.

After Vortigern's death, Ambrosius was conciliatory towards his sons and let them keep their lands in Buellt, Gwerthrynion, Gwent and Powys. Despite this magnanimity, King Pasgen of Buellt & Gwerthrynion later rebelled against Ambrosius and twice attempted to overrun Britain with help from the Saxons and the Irish. The main Anglo-Saxon forces had retired North of the Humber and Ambrosius met Hengist in Battle at Maesbeli and then Conisburgh (Caer-Conan). Later he besieged Octa and Osla at York (Caer-Ebrauc). All were defeated, but Ambroius let them settle their people in Bryneich (Bernicia).

Ambrosius is credited, by Geoffrey, with the building of a monumental stone circle, the "Giant's Ring" (possibly Stonehenge or Avebury) on Mount Ambrius as a memorial to those massacred by the Saxons at the "Night of the Long Knifes" during King Vortigern's reign. He was buried there himself after being poisoned by a Saxon at Winchester (Caer-Guinntguic).

Ambrosius was certainly an historical figure as recorded by his near contemporary commentator, St. Gildas. In his "Ruin of Britain," the monk describes how the Saxons rampaged through the country before they "returned home". Then:

"The remnants (of the British)...take up arms, and challenge their victors to battle under Ambrosius Aurelianus. He was a man of unassuming character, who, alone of the Roman race, chanced to survive the storm in which his parents, people undoubtedly clad in the purple, had been killed. Their offspring in our days have greatly degenerated from their ancestral nobleness. From that time the citizens were sometimes victorious, sometimes the enemy...up to the year of the Siege of Mons Badonicus."

Added to this are the comments of the 9th century chronicler, Nennius, who, in-line with Geoffrey, recorded Ambrosius as one of the chief dreads of King Vortigern. Nennius also describes Ambrosius as a young boy without a father, called to help Vortigern out during the building of his fortress at Dinas Emrys (see Vortigern), a role later taken on by Merlin. He ties the period down by implying that Vortigern's reign had begun by at least 425, and that Ambrosius fought at Guoloph twelve years later. This is most interesting for it poses a bit of a problem. Many people take Gildas' reference to Mons Badonicus to imply that it was Ambrosius, rather than the usually attributed King Arthur, who was the commander at the famous battle of Mount Badon, the decisive British victory over the Saxons around 495-500. In the year 495, Ambrosius would have been at least 74 years old, and it would, indeed, be difficult to imagine a man of this period living to such an age, let alone wielding a heavy sword and leading a mounted charge against the Saxon positions. So what is the solution?

There isn't a definitive one, but some have solved the problem by postulating two men named Ambrosius; the elder, whom Vortigern dreaded, and the younger, the hero of the British resistance of the mid-to-late fifth century and the victor of Mount Badon. This is certainly possible. . .as there seem to have been a number of people with the same name in those days (ie. Maximus, Constantine, etc.). Why not two Ambrosii?

The more likely possibility, though, is that there was just one Ambrosius. Arthur may indeed have been the real commander of the victory at Mount Badon; or perhaps as "the great king among all the kings of the British nation," Ambrosius Aurelianus could have been the aging overall supreme commander of the engagement, with the function of front line battle leader going to a younger man, perhaps Arthur.

Sources
..............

Geoffrey Ashe (1980) A Guidebook to Arthurian Britain.
Gildas Badonicus (c.540) The Ruin of Britain.
Peter C. Bartrum (1993) A Welsh Classical Dictionary.
E.K. Chambers (1964) Arthur of Britain.
Ronan Coghlan (1991) The Encyclopaedia of Arthurian Legends.
Jack Lindsay (1958) Arthur and his Times.
Geoffrey of Monmouth (1136) The History of the Kings of Britain.
John Morris (1973) The Age of Arthur.
Nennius (c.829) The History of the Britons.
John Rhys (1901) Celtic Folklore.
Hugh Williams (1901) Gildas.