Hadrian's Wall, snaking 117km from the Tyne to the Solway Firth, is one of the wonders of the ancient world and one of the most spectacular reminders to modern society of the Roman occupation of Britain.
It was a mammoth undertaking. Soldiers from three legions - the 20th, 2nd and 6th, toiled in the first phase of the wall's construction to throw up a part stone, part turf embankment structure that defined the northern limits of the Romanised world early in the second century.
The legions started their 'grand projet' shortly after Hadrian's visit to Britain around AD122. By the time of his death in 138 the structure was complete.
The 4m high, 1.8m thick wall was by then all stone and likely to have been rendered gleaming white. It continued over water with three impressive, fortified bridges at Chesters on the Tyne, at Carlisle and at Willowford. Small forts were built into the wall at one mile intervals, known as milecastles, and two observation turrets were located between each milecastle. These were later joined or replaced by 16 full scale military forts. And the 34m wide, 9m deep ditch to the north had been joined by an equally impressive vallum to the south.
But what was it for? And why was it there exactly?
The traditional image of thousands of fiercely armed Roman legionnaires lining the heights of the great wall of Hadrian to defend their conquered lands from woad covered Scottish wild men is not currently popular with archaeologists.
They prefer to see it as a power-statement of a frontier, separating the civilised Roman world from Barbarians. It was, in effect, a second century equivalent of the modern wire fence, with border guards and customs posts.
Access through the wall was originally via the single gates to north and south of the milecastles. However, later addition of the 16 forts changed the wall's function. These had more gates to the north than south which suggest they were launching posts for the re-colonisation of what is now Scotland.
By 85AD the Romans had fought their way into the Highlands, as far north as Inchtuthil, under the leadership of the then British governor Agricola. But withdrawal of one of the legions from Britain to beef up Roman military ambitions in other parts of the empire over-stretched resources. The Romans were pushed back, probably by hostile pressure from the Scottish tribes, to roughly the line where Hadrian opted to put his wall.
The fact that the wall bisected the territory of the Brigantian tribe may have been intentional. These nomadic, pastoral people were less impressed by the good life the Romans brought than the tribes in southern England and seemed to have been a constant irritant. But their distress may have led to construction of the southern ditch to protect the legionnaires' backs.
After the Roman venture north of the Antonine line, Hadrian's wall was regarrisoned and acted as the empire's northern boundary until the Romans left Britain. It fell into disrepair. But in devolution week it is interesting to note that a large portion of Hadrian's Wall, near Newcastle, was dismantled in 1745 by General Wade to be used to create solid roads for the English troops to advance to Culloden the following year.