I am fascinated by the Anglo-Saxon period in Britain, and it's curious to me how after ~400 years of Roman occupation and settlement, the culture, building techniques, etc. all reverted to a more basic lifestyle.

From large stone villas and forts to wooden structures and less manufacturing--was there no one left who had the skill or knowledge to build large-scale stone buildings anymore?? I understand that the Legions and the governmental officials returned to Rome, but what about the families who had been there for hundreds of years? Why wouldn't they have stayed? Weren't they by then properly (for lack of a better word) British??