Asser reports that while Alfred was wandering in the marshes of Somerset after his flight from Chippenham, a Viking force – by tradition led by Ubba, a supposed brother of Ivar (and Halfdan?) – had spent the winter in Demetica regione, Dyfed, in west Wales. The host sailed in 23 ships across the Bristol Channel and landed ad Domnaniam. There he fought with ministris regis (thegns of King Alfred) and was killed in front of the fortress of Cynuit.
The Viking tactic had been to besiege the naturally well-defended fortress, and wait until those inside surrendered through lack of food and drink. The leader of the Saxons, said to be Odda, ealdorman of Devon, needed little imagination to work out their likely fate.
The best tactic being to strike while the besieged force was still in good shape, they burst out early one morning and attacked the Vikings with such savagery that few escaped alive. Asser reports that 1,200 of the invaders were slaughtered, and the Battle of Cynuit was a victory for the West Saxons. This was the winter just before King Alfred set out for Eþandune from Athelney, and it’s speculated that this attack on Devonshire and Guthrum’s attack on Chippenham were coordinated by separate arms of the Great Heathen Army.
As for the Anglo-Saxon Chronicle, the Parker Chronicle records only the fleet of 23 ships landing in Devonshire, and that there was a victory where 800 of the enemy were slain. It doesn’t say where they had sailed from, who they fought or where.
Parker: 7 þæs ilcan wintra wæs Inwæres broþur 7 Healfdenes on Westseaxum on Defenascire mid .xxiii. scipum, 7 hiene mon þær ofslog, 7 .dccc. monna mid him. 7 .xl. monna his heres;
The Laud Chronicle says much the same, but adds that in the course of the battle the Vikings’ battle standard – the Raven – was captured.
Laud: 7 þes ilcan wintra wæs Iweres broðor 7 Healfdenes on Westsexum on Defenanscire, 7 hine mon þær sloh 7 .dccc. manna mid him 7 .xl. manna his heres, 7 þar wæs se guðfana genumen þe hi ræfen heton.
[Quotes from Tony Jebson’s excellent online website, a thing to behold).
The much later Annals of St Neots (text Dumville and Lapidge – and, yes, I did buy it, Print On Demand) also mentions the demetica regio and arx Cynuit, and includes more on the making and significance of the captured Viking standard:
 Ibique acceperunt illud vexillum quod Reafan nominant. Dicunt etiam quod tres sorores Hynguari et Hubbe, filie videlicet Lodebrochi, illud vexillum textuerunt et totum paraverunt illud uno meridiano tempore. Dicunt etiam quod, in omni bello ubi praecederet idem signum, si victoriam adepturi essent, appareret in medio signi quasi corvus vivus volitans; si vero vincendi in futuro fuissent, penderet directe nichil movens – et hoc sepe probatum est.
The Raven was said to ‘predict’ victory or defeat by flapping wildly or hanging down, quite still – and ‘this often proved to be true’.
So, where was Asser’s arx Cynuit? Scholars are not 100% certain, but opinion heavily favours Countisbury, close to the present Devon-Somerset border.