Might even have been Roman - they built a lot of earthwork defended sites, even on the march, and there were certainly Roman settlements around Coventry. Given Coventry's central location, it is likely that the Romans in the early years after their invasion would have wanted to secure it from attack by the indigenous population, probably by simple outstation strong-points, probably linked by signal towers.
So far as Angles v Saxons is concerned, I think that's codswallop! Angles and Saxons, while they may have been tribally diverse, were ethnically the same people. Whilst there are ancient documents relating different tribes coming from different locations, there seems little archaeological evidence that they did so; nor is it supported by linguistic evidence. There is possibly more likelihood of an enclave of Celtic people - the pre-AS indigenous people - in any region, though assimilation into AS society was known to have been more evident in the flat lands of the Midlands and East Anglia.
Another point to remember is that if the Angles truly came from Schleswig-Holstein, they would have been both linguistically and ethnically the same as the later Danish invaders. (It is known that Anglo-Saxons and Danes understood each other) Logically, therefore, any differences between Angles, Saxons, Jutes, and Danes would have been subtle, and I think this would support - if there is anything to support - a Celtic enclave, rather than an enclave of Angles in a Saxon area. Incidentally, the Mercian Royal line was reputed to have been of Angle tribal origin.
Perhaps this is Allesleyians trying to claim to be superior to the other inhabitants of Coventry!
