This game is set during the time of the King Arthur legend.
King Arthur is said to have been a Romano-British general who fought against the Barbarian invaders (like Saxons and Angles).
The legend is so varied it gives CA so much diversity to play around with and a topic which fits perfectly in the Total War world.
We even have an abundance of factions who could be involved. The Ebdanians, Picts, Caledonians, Saxons, Angles, Jutes and Geats. One thing they would have to add is a faction in modern day England.
From a historic point of view, the Celts from Ireland and Scotland were invading England and especially the Irish tribes who settled there, then soon after the tribes from continental Europe invaded and settled, one possible reason for the Saxons presence was the tribes in England hiring them as mercenaries to fight against the other invading Celtic tribes. You also had the Irish tribes invading Scotland, which is how it got its name.
So King Arthur would be defending his country from all sides, and with the feel of this game it would be more realistic than wizards and castles that are 500 years too soon. It would be the real King Arthur, defending those flea infested settlements we get with the Northern European factions.
How I would do it:
The campaign would start off defending from Ebdanians and Caledonians, who are possibly allied. Chapter 2 would see those Celtic factions turn on each other, or just treaties breaking down but with a possibility they stay allied. Chapter 2 would also see the Barbarians arrive on the east side of Britain set as nomadic rather than settled factions. This is a crucial time to make friends with other Celts as allies.
The Barbarians would culturally be similar, but also can fight each other as they did soon after settling in England.
No round tables or any of that nonsense, but the real King Arthur in a desperate bid to survive, which really fits with the theme of this game and the idea of surviving before thriving.