It comes as no surprise that one of the least documented and most difficult periods of British history should also be its most keenly studied and hotly debated. Thanks to the continuing and intensive scholastic dissection of our meagre literary sources for the period and the flowering of archaeology since the war, the Dark Ages are not as dark as they were. But the sources are still sufficiently patchy and inadequate and the archaeology still sufficiently ambiguous to provide a playground for amateur historians from the barely literate to the frankly insane - to anyone, in fact, who finds themselves intrigued by the romance of King Arthur. Clearing the clutter of knights, necromancers and nymphs, though, both a serious question and a possible answer emerge. What happened after the last legions departed that ploughed such a diverse jumble into a single, identifiable tilth in which the seeds of nationhood could root and grow? The answer may be that while Constantine I, the Great, made Britain the birthplace of the Christian Roman empire Constantine III, the last usurper, left it a firm fastness in a greater empire still - a fastness that would endure until Rome's return.