Unknown! We do not even know if there was a King Arthur. Most historians believe the legend derives from a dark age warlord 5-6th century who halted the advance of the Saxon invaders across Britain

1) On the River Glein (probably somewhere in Cambridgeshire)

2, 3, 4 & 5) On the River Dubglas in the region of Linnuis (Lincolnshire)

6) On the River Bassas (unknown)

7) In the Forest of Celidon (southern Scotland)

8) Castle Guinnon (unknown)

9) City of the Legion (probably Chester)

10) On the River Tribruit (unknown)

11) At Mount Agned (unknown)

12) At Mount Badon (517AD, probably Barbury Rings) - here the Anglo-Saxons were slaughtered and the threat to Britain that they posed was removed for about 50 years.

Then there is the final battle at Camlann (538AD) where Arthur was betrayed and is mortally wounded. This battle does not appear in Nennius.

Without Arthur, Britain would have been conquered far sooner than it was by the English and this would have made the British Isles a profoundly different place than they are today.

Arthur is mentioned in some early Welsh poems. However, the dates of the passages are somewhat dubious.

One of these is The Gododdin, which says that a certain warrior named Gwawrddur was of great prowess, "though he was not Arthur". Evidently Arthur was known to the poet as a famous warrior. Of course, it is possible that this might not be our Arthur.

The Song of the Graves gives the locations of the graves of many famous heroes, but also contains the stanza:

Bet y March, bet y Guythur,

bet y Gugaun Cledyfrut

anoeth bid bet y Arthur

This can be translated as:

A grave for March, a grave for Gwythur,

A grave for Gwgawn Red-sword;

The world's wonder/ difficutly (anoeth) a grave for Arthur.

Presumably this is a reference to Arthur's gave being unknown, and perhaps to the believe that Arthur lives still in the magic island of Avalon.

An elegy about a hero named Gereint contains the lines:

In Llongborth I saw Arthur's

Heroes who cut with steel.

The Emperor, ruler of our labour.

This is interpreted by some to refer to a band of warriors known as Arthur's Heroes who remained a unit after Arthur's death. However it could be just a late addition to the poem, if indeed the entire poem is not late.

A chronicle called the Annales Cambriae written in its current version at least by 970 contains two Arthurian entries:Year 72 (c. 516) The Battle of Badon, in which Arthur carried the cross of our Lord Jesus Christ on his shoulders for three days and three nights and the Britons were victors.

Year 93 (c. 537) The Strife of Camlann in which Arthur and Medraut fell [and there was death in Britain and in Ireland.] Text in brackets not in MSS. B or C.

They could be older.

The account called Historia Birttonum, usually dated to about the 8th century, contains some Arthurian material:

Then Arthur along with the kings of Britain fought against them in those days, but Arthur himself was the military commander ["dux bellorum"]. His first battle was at the mouth of the river which is called Glein. His second, third, fourth, and fifth battles were above another river which is called Dubglas and is in the region of Linnuis. The sixth battle was above the river which is called Bassas. The seventh battle was in the forest of Celidon, that is Cat Coit Celidon. The eighth battle was at the fortress of Guinnion, in which Arthur carried the image of holy Mary ever virgin on his shoulders; and the pagans were put to flight on that day. And through the power of our Lord Jesus Christ and through the power of the blessed Virgin Mary his mother there was great slaughter among them. The ninth battle was waged in the City of the Legion. The tenth battle was waged on the banks of a river which is called Tribruit. The eleventh battle was fought on the mountain which is called Agnet. 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, and in all the wars he emerged as victor. And while they were being defeated in all the battles, they were seeking assistance from Germany and their numbers were being augmented many times over without interruption. And they brought over kings from Germany that they might reign over them in Britain, right down to the time in which Ida reigned, who was son of Eobba.

The are some variations in this text in different manuscripts.

These are arguably the oldest Arthurian texts. There are other Welsh texts of dubious date.