Why did the Romans help these people, if that’s what they did? Any reason you might image could be right.
Maybe Severus was a decent sort. Maybe Severus was up north and this was left to an underling or to the soldiers who could have been friendly with the locals. Or intermarried with the locals. Or maybe they were the locals.
They were probably allies of Rome’s to begin with. The Romans occupied Lowland Scotland, the Emperor had other priorities than finishing up the conquest, so the army built the Antonine Wall. Later they built and pulled back to Hadrian’s wall. Still later they reoccupied Lowland Scotland and reconstructed parts of the Antonine Wall. Gosh, it’s like modern politicians were in charge...
Did Septimius die of Septicimia?
Round huts built by Celts = Peel towers. Ground floor = barn, first floor = home, second floor = grainary. Primitive castle.
I think the Romans helped these people because they were local recruits. Celts.