It is much more than a mere fortification; it is a dividing line between so many things. Between civilisation and barbarism, between safety and danger, between the tamed and the wild, between the settled country and the outland which was too hot to handle and not worth fighting over anyway, between "us" and "them'; we have seen, in our own time, how a wall across Berlin is a barrier of the spirit as much as of bricks and mortar. Hadrian’s Wall has lasted immeasurably longer than the Berlin wall ever will, and in its way it lives in the minds of people who have never even heard of it or seen it, or if they have, think of it only as an interesting relic which stands at an inconvenient distance from their cars and coaches.
Although any Northern Englishman can answer in five words the question: why was it built? ("To keep the Scots out"), there is still learned dispute on the point. The suggestion that it was erected to keep the inhabitants of England in has been advanced, not altogether frivolously; so far as the Wall was there for effect, it certainly operated in both directions. The layman, looking at its imposing size — it was originally about twenty feet high and ten wide, and although no part of it today is as tall as this, it is still an awe-inspiring barrier — may be excused for thinking it was a defensible castle wall on a gigantic scale. In fact, it was not intended to be a Maginot Line. As Viscount Montgomery has pointed out, it was a deterrent rather than a defence, which could never have resisted a well-organised invasion, and indeed the wild men from the north overran it and its chain of castles and platoon strongpoints on at least three occasions.
But it was not an obstacle that any raider could take lightly; even if he succeeded in crossing it, and escaping the attention of Roman sentries who were never more than half a mile away, and usually no doubt a good deal closer, he still had the problem of returning with whatever he had lifted on the southern side. The Wall was, in effect, a glorified police beat seventy miles long, manned by hard men who must have detested it. [The Steel Bonnets: The Story of the Anglo-Scottish Border Reivers (1971), pp. 13-14]