What were the transitions, How long did each transitional period last.
The western Empire was totally overrun, and very little Roman culture was left. Many families fled abandoning estates, especially the aristocracy. It was quite a sudden transition, but many buildings survived and were used by the occupiers. The technology and knowledge that the Romans had left in each province was lost, and you can imagine how throughout the dark ages and well into the middle ages, people must have marvelled at the skill apparently needed to have left the elegant villas, bridges, aqueducts behind. Stone building was a rare skill for centuries afterwards, and concrete had to be rediscovered. In England for example, the main Roman towns became the main cities of the Anglo Saxons. Wintanceaster (Winchester) was the court of Alfred the Great, previously Venta Belgarum. Lundenwic (London), used to be Londinium. York, Eboracum. The crumbling fortifications were supplemented with simple palisades and utilized as 'burhs'. I could certainly tell you much more about this time, but please be more specific, in case I end up writing a book. And I would too.