The Britons, also known as Celtic Britons or Ancient Britons, were Celtic people who inhabited Britannia during the Roman occupation.
The traditional view that the Celtic Britons originally migrated from the continent, mostly across the English Channel, with their languages, culture and genes in the Iron Age has been considerably undermined in recent decades by the contention of many scholars that Celtic languages had instead spread north along the Atlantic seaboard during the Bronze Age, and the results of genetic studies, which show a large continuity between Iron Age and older British populations, suggesting trans-cultural diffusion was also very important in the introduction of the Celtic languages.
During the events in the game, the Britons were ruled by the benevolent, and popular King Oswald, and the Roman Governor. However, Nero secretly had the governor murdered, and replaced him, with his two sons whose actions sparked an uprising that eventually made its way to Rome itself.
Celtic Britons on Wikipedia