The Celtic Christian monk, Gildas in De excidio et conquestu Britanniae (The Loss & Conquest of Britain) AD 545 mentions  the Battle of Badon occurring in about AD 500 (but does not mention Arthur) which  in 730 Bede dated to 493. Thus Arthur's life may have overlapped Gildas'¹⁷. 
About AD 950-1000 the Annales Cambriae records Arthur's victory at Mt. Badon and also referred to the Battle of Camlann in AD 537 "in which Arthur and Medraut fell" 

Geoffrey of Monmouth a Welsh cleric gives the earliest story of Arthur's life in Historia Regum Britanniae (History of the Kings of Britain) in 1135¹¹, but Geoffrey is now much maligned for his "embellished" history and probable fabrications.⁶ However Geoffrey is the first to popularise the legend. 
In the 1190's Hugh De Morville of  Cumberland & Westmorland is said to have taken the local story of Arthur in written form to the Continent when he replaced King Richard of England as a hostage of the Austrians. This work influenced Ulrich Von Zatzighoven who in turn influenced Chretien De Troyes
who wrote Morte D' Arthur which reintroduced Arthur into Britain as a medieval romance. Marie Capet the daughter of Eleanor of Aquitane and Louis VII Capet was a patron of Chretien De Troyes who at Marie's suggestion composed the romance of Sir Lancelot and Queen Guinevere.¹⁸ Both Marie and Queen Eleanor wanted a 'Court of Love' and set down the rules for such, this led to the Romantic period in Europe and   paved the way for courtly manner and even some of today's attitudes of how men should behave towards women. Arthur Plantagenet, son of Geoffrey Plantagenet, son of Eleanor and Henry II of England was named by Constance of Brittany, his mother, after King Arthur, who in legend was King of the Bretons. Arthur Plantagenet was probably killed by King John to remove him from contention as a future king of England.

Morville had his castle at Eamont Bridge and it is here that a raised earthen structure is found called "The Round Table". This site seems to be prehistoric but was used as a meeting point between the Scots and English in Athelstan's time. Near Plumpton Wall was a lake, now drained, which was reputed to be where the 'Lady of the Lake' appeared.
It was from Geoffrey of Monmouth's works that 
Chaucer (ca. 1340-1400) mentioned Arthur:-
"In old dayes of the King Artour
Of which that Bretons speken gret honour,
All was this land fulfilled of faeries:
The elf-queen with hire jolie company
Daunsed full of te in many a grene mede;
This was the old opinion as I rede.
I speke of many hundred yeres agoe,
But now can no man see non elves mo".

The story of Arthur found its way to France during the "Chivalrous Period" and returned to England through Malory (Mallory) written in the vernacular.
Sir Thomas Malory wrote a work Le Morte d'Arthur concerning Arthur in the 1460's which was published by William Caxton in 1485, but a question remains- who was Thomas Malory? He could have been: 
i) A Welshman or 
ii) A person living at Newbold Revel in Warwickshire who was a criminal  & who appears to have written "Le Morte d'Arthur" whilst in Newgate prison. No original Ms. was ever found, only Caxton's printing which was a translated work from a French Ms. in Dick Whittington's library in Greyfriar's monastery. 
iii) In 1934 W.F. Oakeshott located a Mallory Ms. at Winchester College. 
iv) In 1966 William Matthews discovered another Thos. Mallory Ms. 
The discovery of this Ms. suggests Thomas Mallory was the son of William Mallory of Hutton, Yorkshire and Dionisia of Studley Park near Fountains Abbey, nr. Ripon, Yorkshire. From this lineage it was shown that Mallory died before 1485. Thomas Mallory drew upon French texts written in a northern English regional dialect (English was gradually becoming standardised) 
At Ribston Hall, Wetherby, Yorkshire, a Ms. was found in 1945 and bought by Cambridge University. It had been found in a trunk with a charter and other documents of the Mauleverer family who owned the estate and intermarried with the same Yorkshire families as the Mallory's of Hutton & Studley.
The Ms. contains the "History of the Holy Grail"* and "Merlin" This Merlin version of Merlin corresponds very closely to Mallory's translation. 
*The chalice used by Christ at the last supper supposedly brought to Glastonbury by Joseph after Christ's death. Joseph may have visited on occasions before on tin trading excursions with Jesus as a boy but this may be the monks of Glastonbury again trying to create a myth. Some believe that the Rosslyn Templar Chapel  at Rosslin houses the chalice beneath its floor.

Local tradition says this was part of Camelot where Arthur had his court and lived with Queen Guinevere.(both these names are French and do not bear much relation to Celtic nomenclature). Merlin was reputed to have lived in a sea cave under the castle. Cornish tradition places the battle of Camlann at Slaughter Bridge on the Camel River¹⁷.
 

Castles reputed to be Arthur's:
Pendragon Castle at Mallerstang, Cumbria: 
Cumbria was a Welsh-British haunt & this is a reputed castle of Arthur. Saxton's map of Westmorland (1576) shows Mallerstange Forest and Pendragon Castle at the eastern boundary of the forest on the river Eden. 
The Parish church is built on the site of a Saxon church which contains relics including the "Loki Stone" from the 700's. Loki was the Norse god representing a bound devil, this is one of only two known in Europe¹³. 
Castle Hill (Camulodunon) near Almondbury, Yorkshire. The name may identify with "Camelot" Cam is also found in the Roman Camulodunum for Colchester and means "crook bank". 
A Brigantian hill fort of the Iron Age Celtic Welsh Britons, attacked and probably overthrown by the Romans. 
Richmond Castle in Yorkshire where tradition  says King Arthur and his Knights of the Round Table sleep¹⁵. A possible site for the Brigantes to survive after the Romans left Britain. 
Cadbury Castle in Somerset, another Iron Age hill fort over-run by the Romans early in their occupation of Britain shows evidence of occupation in the 500's.

Eamont Bridge, Cumbria here an earthworks is named the "Round Table". Later in 927 Aethelstan received the submission of the Scots and Northumbrians here
. 
Yeovil in Somerset revealed during excavations a large settlement of the 500's that has been described as a possible contender for Camelot, Arthur's legendary court. 

A Scottish castle near Ayr.

Cybwr in South Wales.

Brittany, France. 

Burial sites for Arthur have been suggested at Arthuret, Glastonbury, Bridgend, Caer Melyn & Baschurch.
Arthuret [Arthur's Head]:
In 1990 an American historian, Professor Norma Goodrich  of Columbia University, New York, claimed to have discovered the burial place of Arthur in Scotland near the English border, at Arthuret, the Roman Castra Exploraturum. Arthuret is near Camboglanna where it has been suggested Arthur fought his last battle. Prof. Goodrich believes she has traced the court of "Camelot" to the ruins of a Scottish castle near Ayr. 
According to information collated by the late Professor N. Chadwick of the University of Wales the name Arthuret means "Arthur's Head". This was deduced after studying 6th century Irish & French epics in which the hero's head was generally buried with the face to the foe. A high point of land near the Church of St. Michael and All Angels in the parish of Arthuret is consecrated ground⁹. However Christopher Saxton's map of Westmoreland (1576) gives the name as ARTRUTHE which if phonetically spelled would sound more like "Arthur"¹⁴. 

Glastonbury:
In the cemetary of the monks on the Isle of Avalon, twelve miles from Cadbury Castle. According to Gerald of Wales there were two stone pyramids with much eroded inscriptions on them. A skeleton lay buried  between these two pyramids at a depth of 16 feet. The skeleton lay in a hollow oak coffin. Two thirds of the coffins length was taken up by what was purported to be Arthur's body and the lower third contained his second wife Guinevere retaining traces of blonde hair. At the head of the coffin lay a lead cross under a stone with the inscription 
According to Camden:
"HIC JACET SEPULTUS INCLITUS REX ARTHURIUS IN INSULA AVALONIA" 
According to Leland: 
"HIC JACET ARTURUS, FLOS REGUM, GLORIA REGNI, QUEM MORES, PROBITAS COMMENDANT LAUDI PERENNI" 
According to Ruben: 
HIC JACET ARTHURIS, REX QUONDAM REXQUE FUTURUS 

It is now considered that the suggestions for Glastonbury being the site of Arthur's burial are hoaxes  perpetrated by the medieval monks who wished to obtain money for building during the Crusades. Others however have cast doubt upon this assertion saying that the monks never used this as a means of procurring money.
 

Now the general thesis is that Arthur (Latin: Arthurius) was a Romano-Briton of the 500's who welded the Romano-Britons, 
 and Welsh Celts against the Anglo-Saxons after the withdrawal of the Roman armies, and that he may have existed in fact & deed. 
His successes in battle were first popularised by wandering troubadours. 
i) Portchester in 501 under King Ambrosius aurelianus, Arthur may have lost a battle here, near Portesmutha [Portsmouth]. 
ii)The Battle of Camlann 537 A.D.² This site is identified as the Roman fort of Camboglanna or Birdoswald ¹⁶on Hadrian's Wall in Northumberland. The Roman name Camboglanna or "crook bank" which may identify with "Camelot"* lies near a stream called King Water draining from near Midgeholme Moss.
Arthur is supposed to have died here in 537 fighting the Saxons³
Later in 573 A.D. the Battle of Armterid was fought a few miles to the N.W. of Arthuret²
[*note: The term "Camelot" is an invention of the French poet C. de Troyes in the 1100's]  

King Arthur the film 

Geoffrey recorded that Arthur lived in N.W. Wales (Gwynedd) in the 400's and was titled "Prince of Gwynedd", his father was Uther Pendragon# of Gwynnedd. Nennius in his "Historia Brittanum" in 830 A.D. recorded that the kings of Gwynned descended originally from Cunedda a warrior from Gododdin the kingdom of the Votadini tribe in S.E. Scotland. 
[# note: Uthr = terrible; Pendragon = head dragon] 
The dragon is part of the Welsh flag today. 
The Votadini took control of Gwynedd and the Cunedda family became the kings. 
Archaeology verifies that N.W. Wales was colonised by warriors from Gododdin in the 460's, Votadini pottery discovered in Gwynedd dating from the early 400's had been recovered. 
The identifying family prefix Cun- (Welsh=Cyn-) of the Cunedda family are found on tombstones and in the genealogies of Gwynedd. 
The Gododdin, a poem now in the Public Library in Cardiff was probably composed in the early 600's. One passage praises the courage of a hero saying "although he fought bravely "he was no Arthur". This war poem was written by the Vodadini suggesting he had been a member of their tribe.
Powys: 
Cuneglasus was king of Powys at the time of Gildas (ca. 545). Powys comprised the present regions of Central Wales & the West Midlands, the state of Powys adjoined Gwynedd. Cuneglasus was descended from Cunedda. In the 400's Viroconium (Wroxeter in Shropshire) was the capital of Powys. In 1967 an inscribed stone was discovered commemorating king Cunorix (Cunedda family). This stone was dated to ca. 480 i.e. he died about this time, this indicates that the Votadini were ruling then. 
From archaeological evidence it is known that Viroconium was wealthy & powerful at this time. By the 430's Viroconium was being rebuilt in a highly sophisticated fashion. A massive winged mansion was built, this could have been a palace of post Roman chieftains. The city was not abandoned until 520 A.D. There was no threat from the Anlo-Saxons for perhaps 20 years, and Powys did not fall to the Anglo-Saxons until the 650's. It is possible Cunorix may have been succeeded by Arthur, but Geoffrey of Monmouth says Arthur died in 537 at the Battle of Camlann. 
Modred:
Also called Maglocunus popularised as the evil Mordred Arthur's treacherous nephew who killed Arthur in the battle  at Camlann according to Geoffrey of Monmouth. The Annales Cambriae records that Arthur & Mordred did fall at the battle. 
Five miles east of Dolgelly is a remote, bleak valley called Camlan it lies on the old border of Powys & Gwynned. 
A poem⁷ written ca. 850 now in the Bodleian Library at Oxford, mentions Arthur & Cynddylan being "heir of the Great Arthur". It states that the body of Cynddylan was buried at the "Churches of Bassa". It may be that Arthur was buried earlier here. Today this is called Baschurch a village nine miles N.W. of Shrewsbury.
On the edge of the village is "The Berth" a fortified hillock surrounded by marshland and linked to the mainland by a gravel causeway. In an archaeological excavation in 1962-3 pottery from the 500's was uncovered. Most of the Berth has not been excavated, it has been suggested that one of the mounds may contain the remains of Arthur⁸