Emperor Hadrian didn’t order the wall built because he was afraid of northern invasion. Truth is no part of the wall was impenetrable – a concentrated attack at any single point would have surely breached it – but was meant to mark the border as though to say that the Roman Empire would extend no further. By drawing a physical boundary, the Romans were also tightening their grip on the population to the south – for the first time in history, passports were issued to citizens of the empire, marking them out not just as citizens but, more importantly, as taxpayers.
But all good things come to an end. It’s likely that around 409, as the Roman administration collapsed, the frontier garrisons ceased receiving Roman pay. The communities had to then rely on their own resources, gradually becoming reabsorbed into the war-band culture of the native Britons – for some generations soldiers had been recruited locally in any case.