Arthur seems to have fought a series of battles against the same enemies, or at least enemies under the same leader, and after the last one he drove them in to the sea never to be seen again.

50. Then it was, that the magnanimous Arthur, with all the kings and military force of Britain, fought against the Saxons. And though there were many more noble than himself, yet he was twelve times chosen their commander, and was as often conqueror. The first battle in which he was engaged, was at the mouth of the river Gleni. The second, third, fourth, and fifth, were on another river, by the Britons called Duglas, in the region Linuis. The sixth, on the river Bassas. The seventh in the wood Celidon, which the Britons call Cat Coit Celidon. The eighth was near Gurnion castle, where Arthur bore the image of the Holy Virgin, mother of God, upon his shoulders, and through the power of our Lord Jesus Christ, and the holy Mary, put the Saxons to flight, and pursued them the whole day with great slaughter. The ninth was at the City of Legion, which is called Cair Lion. The tenth was on the banks of the river Trat Treuroit. The eleventh was on the mountain Breguoin, which we call Cat Bregion. The twelfth was a most severe contest, when Arthur penetrated to the hill of Badon. In this engagement, nine hundred and forty fell by his hand alone, no one but the Lord affording him assistance. In all these engagements the Britons were successful. For no strength can avail against the will of the Almighty

56. At that time, the Saxons grew strong by virtue of their large number and increased in power in Britain. Hengist having died, however, his son Octha crossed from the northern part of Britain to the kingdom of Kent and from him are descended the kings of Kent. 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. He was the first king in Bernicia, i.e., in Berneich.

 None of the places named is easily recognizable but they can be interpreted as describing a series of invasions or raids by the English as they sailed around the coast of the Southwest of Britain, from Exeter round Lands End to Barnstaple Bay, where the final battle, the siege of Mount Badon, took place. 

'Saxon ships were for a period free to distress the peoples of the Atlantic coasts. The Irish Annals place the 'second Saxon raid on Ireland' in 471. Sidonius called them 'masters of the sea whose raids on the Biscay coast were deterred by a Gothic admiral's "half naval, half military duty of coasting the western shores, on the look out for the curved ships of the Saxons."

'The Saxons plundered on both sides of the Channel, attracting most notice in the thirty years or so after 460, when they controlled the Sussex and Hampshire havens, raiding the Atlantic coasts of Gaul and Ireland as well. The Age of Arthur - John Morris

Reading a list of various authorities interpretations of the locations described by Nennius makes very clear the fact that there are no universally accepted interpretations of the sites of any of the battles.

THE BATTLE OF VELLA N-DRUCHAR.

Vellan (mill), druchar (wheel)

THE Sea Kings, in their predatory wanderings, landed in Genvor Cove, and, as they had frequently done on previous occasions, they proceeded to pillage the little hamlet of Escols. On one occasion they landed in unusally large numbers, being resolved, as it appeared, to spoil many of the large and wealthy towns of Western Cornwall, which they were led to believe were unprotected. It fortunately happened that the heavy surf on the beach retarded their landing, so that the inhabitants had notice of their threatened invasion.

That night the beacon-fire was lit on the chapel hill, another was soon blazing on Castle-an-Dinas, and on Trecrobben. Cam Brea promptly replied, and continued the signal-light, which also blazed lustrously that night on St Agnes Beacon. Presently the fires were seen on Belovely Beacon, and rapidly they appeared on the Great Stone, on St Bellarmine's Tor, and Cadbarrow, and then the fires blazed out on Roughtor and Brownwilly, thus rapidly conveying the intelligence of war to Prince Arthur and his brave knights, who were happily assembled in full force at Tintagel to do honour to several native Princes who were at that time on a visit to the King of Cornwall. Arthur, and nine other kings, by forced marches, reached the neighbourhood of the Land's-End at the end of two days. The Danes crossed the land down through the bottoms to the sea on the northern side of the promontory, spreading destruction in their paths. Arthur met them on their return, and gave them battle near Vellan-Druchar. So terrible was the slaughter, that the mill was worked with blood that day. Not a single Dane of the vast army that had landed escaped. A few had been left in charge of the ships, and as soon as they learned the fate of their brethren, they hastened to escape, hoping to return to their own northern land. A holy woman, whose name has not been preserved to us, "brought home a west wind" by emptying the Holy Well against the hill, and sweeping the church from the door to the altar. Thus they were prevented from escaping, and were all thrown by the force of a storm and the currents either on the rocky shore, or on the sands, where they were left high and dry. It happened on the occasion of an extraordinary spring-tide, which was yet increased by the wind, so that the ships lay high up on the rocks, or on the sands; and for years the birds built their nests in the masts and rigging.

Thus perished the last army of Danes who dared to land upon our western shores.

King Arthur and the nine kings pledged each other in the holy water from St Sennen's Well, they returned thanks for their victory in St Sennen's Chapel, and dined that day on the Table-men.

Merlin, the prophet, was amongst the host, and the feast being ended, he was seized with the prophetic afflatus, and in the hearing of all the host proclaimed--

From - Popular Romances of the West of England collected and edited by Robert Hunt [1903, 3rd edition]