(i) Because the Romans withdrew from Britain, the Britons organized their own defense and were temporarily successful, thus generating the Arthurian legend.
(ii) Independent kingdoms were formed in former Roman territories in Northern Europe. In the Andersons' fiction, an independent Armorican king is the younger brother-in-law of the last King of Ys.
(iii) Alaric the Visigoth not only harassed the Empire but even captured and sacked Rome in 410, just a few years after the events of the Tetralogy.
(iv) The Imperial capital had in any case moved from Rome to Milan, then to Ravenna. The second move was for a quick exit route to Constantinople in the event of further barbarian incursions.
(v) Britons emigrated en masse to Armorica, changing its name to Brittany. Latin became Italian, French, Spanish etc. The world as we know it came into being.