Romano British Way of Life

How quickly and how hard did the British economy turn to mush? What happened to civil and political authority and governance while this happened? Where are we with Christianity and crucially, where are the angles, Saxons and Dukes, and Franks and Frasions while we're at it? The written evidence is sparse and difficult to read. St Patrick was probably a child in the early 5th century. His life doesn't tell us a lot about Britain, but it does tell us something.