The Battle of Arfderydd or Arderydd is one of only four battles listed in Annales Cambriae for the sixth century. Two of the others are Arthur's battles at Badon and Camlann, so it's in illustrious company. What do we know about the battle?
573 The battle of Arfderydd between the sons of Eliffer and Gwenddolau son of Ceidio; in which battle Gwenddolau fell; Merlin went mad.
580 Gwrgi and Peredur sons of Elifert died
--Annales Cambriae, available online
Gwendoleu a Nud a Chof meibyon Keidyav m Arthwys m Mar m Keneu m Coel
--Bonedd Gwr y Gogledd (Descent of the Men of the North), available online
Gvrgi a Pheredur meibon Eliffer Gosgorduavr m Arthwys m Mar m Keneu m Coel
--Bonedd Gwr y Gogledd (Descent of the Men of the North), available online
Gurci ha Peretur mepion eleuther cascord maur map letlum map Ceneú map Coylhen.
--Harleian Genealogies, available online
...Corvan, horse of the sons of Eliffer, bore the second Horse-Burden: he carried on his back Gwrgi and Peredur and Dunawd the Stout and Cynfelyn the Leprous(?), to look upon the battle-fog of (the host of) Gwenddolau (in) Ar(f)derydd
Three Faithful War Bands
...and the War-Band of Gwenddolau son of Ceidiaw at Ar(f)derydd, who continued the battle for a fortnight and a month after their lord was slain. The number of each one of the War-Bands was twenty-one hundred men
--Red Book of Hergest, available online
Merlin, or Myrddin, features in the poems Afallen (Apple Trees) and Oianau (Greetings, Piglet) in the Black Book of Carmarthen, in which he says he is hiding from Rhydderch and mourns that no-one now honours him after the death of Gwenddolau at Arderydd. This is mystical poetry, not a historical narrative, and should be interpreted with caution, but it may draw on older traditions.
The late medieval manuscript called Lailoken and Kentigern says that the mad prophet Lailoken was also called Merlin, and was living wild in the wilderness following a terrible battle "fought on the plain between Liddel and Carwannok" (source: Wikipedia page on Myrddin Wyllt).
Annales Cambriae is clear that the battle of Arfderydd was fought between the sons of Eliffer, named a few entries later as Peredur and Gurci, and Gwenddolau son of Ceidio. All three protagonists and their respective fathers are also listed in the genealogies, and appear associated with the battle in the Triads. This may not be independent confirmation, since it is possible that the sources could have copied each other, but three sources all consistent with each other is impressive.
The Merlin poetry has Merlin hiding from Rhydderch, presumably Rhydderch Hael, who was king of a territory centred on Alt Clut, modern Dumbarton, in the late sixth century. At first glance this might indicate that Rhydderch was the winner of the battle, which would either mean he was allied with Peredur and Gurci or would be inconsistent with the Annales Cambriae. However, the poetry does not explicitly say that Rhydderch fought or won the battle (as far as I can make out), only that Rhydderch is Merlin's enemy. This is equally consistent with Rhydderch having opportunistically moved in to part or all of Gwenddolau's former territory after the battle without having fought in it, or with Merlin having wandered into Rhydderch's territory in his madness.
It is worth noting that although the genealogy brackets Gwenddolau with two brothers, named Nud and Chof, in the same way as Peredur and Gurci are bracketed together in their genealogy, there is no mention of Gwenddolau's brothers at the battle. It could simply be that no-one remembered to mention them, or that they were dead before the battle took place, or that they were elsewhere at the time, or that for whatever reason they chose not to fight alongside their brother.
All the participants appear in the genealogies of the 'Men of the North'. Rhydderch was king of the area around modern Dumbarton, and Peredur is traditionally associated with York (see article on Peredur for the rationale). It therefore seems likely that the battle of Arfderydd was fought somewhere in the region that is now southern Scotland and northern England.
If the manuscript Lailoken and Kentigern can be trusted (which is a big 'if', as it dates from the fifteenth century and is thus nearly a thousand years after the event), it may provide a more detailed clue to the location. It says, "fought on the plain between Liddel and Carwannok". Liddel must surely be the Liddel Water, which runs south-westwards from the hills at the northern end of the Pennine chain to join the River Esk on the coastal plain at the head of the Solway Firth, and which was later famous, or rather infamous, as the worst haunt of the Border Reivers. As far as I know Carwannock has not been identified, but the first element 'Car' is the Brittonic 'Caer', meaning a fort. There are plenty of Roman forts in the area, including the chain along Hadrian's Wall and outpost forts north of the Wall, and there may well have been other fortified places. This would place the battle somewhere on the Solway plain, perhaps at a strategic crossing of the River Esk or the Liddel Water, which is a plausible sort of location for an important battle.
Arthuret House, on the River Esk just south of Longtown, is the traditional site ascribed to the battle. It is on the plain, not far from Liddel Water, and could count as 'between Liddel and Carwannok' if Carwannok is to be identified with one of the forts on Hadrian's Wall. Nearby Carwinley, which stands on the Liddel Water not far from the site of the Roman fort at Netherby, has been suggested to derive from a name something like 'Caer Gwenddolau'; if correct, this derivation is further support for locating Arfderydd somewhere in this approximate area.
The cause of the battle is unknown. The battle was evidently important enough for the compiler of the Annales Cambriae to consider it worth recording. The 'twenty-one hundred men' of the Triads is probably a poetic convention, though it may indicate that the forces involved were remembered as being unusually large. Similarly, it is unlikely that the defeated warriors literally continued the battle for six weeks (!), but that may indicate a tradition that the battle was exceptionally hard fought. All of this is consistent with it having been a major battle, either in numbers or because it was politically decisive in some way, or both.
The 'Men of the North' genealogies show Peredur and Gurci as first cousins to Gwenddolau, suggesting that the battle was a dispute between two branches of the same family. Perhaps it was a sort of sixth-century Wars of the Roses, with each side claiming the other's inheritance. It is worth noting that there is no mention whatsoever of Arfderydd having been fought against 'Saxons' or other invaders; it appears to have been a strictly family affair.
Gwenddolau clearly lost both the battle and his life. Peredur and Gurci evidently survived, since the Annales Cambriae enter their deaths seven years later in 580. So we can reasonably infer that the immediate outcome was that Peredur and Gurci won. Given that the battle was considered worth remembering, it may be that it also had longer-term consequences that altered the political balance in the area. The genealogy ends at Gwenddolau's generation, suggesting that either he left no heirs or that his descendants did not reclaim their territory.
The area over which Gwenddolau ruled is not known. It is a reasonable inference (though not a certainty) that his territory was somewhere in the region of the battle, in what is now north-west England and/or south-west Scotland. He presumably controlled substantial resources, if he could maintain a large warband, and therefore it can also be reasonably inferred that his territory was large or wealthy or both. If this is the case, the prospect of gaining it as a prize may have contributed to the motivation for the battle.
If Peredur and his brother were indeed based in York (see article on Peredur for the rationale), this raises some interesting questions. What were the kings of York doing fighting a battle on the other side of the country? Was there some sort of regional overlordship in dispute, or were they trying to extend their territories, or were they simply intent on destroying Gwenddolau for personal reasons that had nothing to do with claiming territory? If the latter, it may explain why Rhydderch was present in the area to hound the insane Merlin; he may have quietly moved into a gap left after the York kings went home. If Peredur and Gurci took over Gwenddolau's kingdom, one wonders how the logistics would have worked to control a territory sprawling from York to the Solway with the Pennines in the way, unless one of the brothers ruled it as a separate sub-kingdom.
Interestingly, a generation or so after the battle of Arfderydd the kingdom of Rheged appears to have emerged as a powerful force in much the same area. Its king, Urien, is remembered in Historia Brittonum as having almost destroyed the Anglian kingdom of Bernicia, at a date some time before Aethelric became king of Bernicia in 593. The location of Rheged is not known with any certainty. It was presumably somewhere in the North, since Urien appears in the 'Descent of the Men of the North' genealogies and since his recorded enemies and allies are all associated with the region. It was not on the east coast, since that area is accounted for by Bernicia and Deira, so that leaves a gap on the map in the area that is now north-west England and/or south-west Scotland, and it seems logical to place Rheged there. How far its boundaries extended is open to argument, and in any case probably varied according to the military success of its kings relative to their neighbours. This is the same area as the location of the battle of Arfderydd. Which could be pure coincidence, or could perhaps indicate that the battle of Arfderydd was connected with the later rise of the kingdom of Rheged, either directly or by creating a gap into which a new dynasty could move.
Another interesting observation is that the traditional site of the battle at Arthuret House is less than 20 miles from Birdoswald Roman fort. As discussed in another article 'Birdoswald Roman Fort: post-Roman activity on the site', someone in post-Roman Birdoswald built two successive massive timber halls on the site of the north granary, which would be consistent with occupation by a local ruler with control of substantial resources. The halls cannot be precisely dated, and the excavator suggests that occupation may have lasted on the site until around 520 (see article 'Birdoswald Roman Fort: dating the post-Roman use of the site' for rationale). If this date is correct, the halls on the north granary site (though not necessarily the whole fort) would have been long abandoned by the time of the battle of Arfderydd. However, if the halls lasted longer than estimated, which is possible given the absence of dating evidence, this raises the question of whether Birdoswald and/or its ruler had any connection with the battle of Arfderydd. A major chieftain's hall and a major battle within 20 miles of each other may not be entirely coincidental. Could Birdoswald have been the seat of Gwenddolau or his family? If so, one could speculate that the battle at Arfderydd may suggest a context for the end of the sequence at Birdoswald. I need hardly say that this is speculative and that other interpretations are possible.
Annales Cambriae, available online
Bonedd Gwr y Gogledd (Descent of the Men of the North), available online
Harleian Genealogies, available online
Welsh Triads, Red Book of Hergest, available online
Arthuret House, near Longtown