Register here - as a registered user you get more features and fewer ads.
I think you're right that, whoever built it, it's something of an enigma. Why was it built and why was it constructed the way it was? I think it's more apt to compare it with the Antonine Wall than Hadrian's though. This too was a turf wall and would have experienced the same problems.
But taking Hadrian's and the Antonine Walls into consideration, it should be remembered that they are no longer just seen as Walls to keep the enemy out but as means of policing who came through their gates and, as you say, a line to say "everything this side is our territory", especially in the case of the Antonine.
If we, for a moment, assume it was built by in the 2nd or 3rd centuries does it then help us to know why Wat's Dyke was constructed in the 5th? In fact, this date may only refer to the southern section, which means it was either started at this time or completed at this time. So it was either constructed by Post Romano-Britons or the remaining legions from Chester? Why? Was it too meant to have stretched down to the Severn Estuary but never got completed or was this as long as it was ever going to be? Was it constructed to keep out the invading Scotti in the 360's or was it at the request of Theodosius I after he'd regain control of Brittania following the Barbarian Conspiracy for the same reason? The latter may not make sense as Roman forts in north-west Wales were brough back in to commission at this time. There is certainly evidence of renewed activity at Chester. It's also the kind of thing Roman Theodosius' historians would have bragged about.
It's a little hard to judge if the Romans had any remaining legions in the area in the 390's as the Notitia Dignitatum, compiled in 395, mentions no miltary units for the whole of Wales. Could this be why it was constructed?
Wat's Dyke seems to get overlooked in all this. Even brochures put out by the Shropshire Tourist Board still say in was built in the 600's and the Clwyd Powys Archaeological Trust still call it Saxon on their website. We now know it was either very late or post Roman, but no one is talking about it or using it as the closest example. It's almost as if they don't mention it, it will go away. Maybe it's because they think this find a one off and not worth taking further?
So even if Offa's Dyke can be argued over and can't be agreed upon until further archaeology either rules its Roman origins out or proves them, then there's plenty to be going on with on Wat's Dyke. I'm sure this is being done, somewhere in some University. But archaeology and history being what they are it will be many years until it filters its way down to the general public who will go on calling it Saxon for many years to come.
Something is not right. This message is just to keep things from messing up down the road