I would like to explore the feasability of castle building occuring earlier than the 9th C. They surely could not have been beyond the scope of the Romans, they were economical of manpower, the Romans had concrete. Castle building east of the Rhine might have pacified the Germanic tribes, it could have allowed the Romanisation of scotland, one of the things that strikes me about the later roman empire is the weakness of fortification. By Castles i mean including towers, motte and bailey, its a strange oversight that the Romans missed this. Well placed high stone/concrete towers at strategic rivers crossings and other points could have been very effective. I think im correct in saying that many Roman civil engineering projects were far bigger constructions than a lot of medieval castles/towers/forts.