This is my understanding (based on a seriously incompetent and internally contradictory first-year lecture) of the defensive strategy employed by the Roman Empire until around the third century. It applies to Hadrian’s Wall and probably the more northerly, less successful Antonine Wall.
GREEN LINE: The frontier. Generally built along a natural barrier; if one wasn’t available, they built a wall, as here.
GREEN BOXES: Frontier forts. Garrisons of variable quality, usually auxilia. What we’d nowadays consider second-line troops. Do sentry duty on the walls and have fast communications to rear lines (despatch riders, runners, beacons, smoke signals.)
BLACK LINES: Roman roads. Straight.
RED BOXES: Forts with garrisons made up of professional legionaries. Very well trained, equipped and disciplined soldiers with good officers and generally lots of combat experience. Bad enough dudes.
The frontier has several purposes. It’s a massive prestige thing, the largest man-made object locals would have ever seen (…apart from Hadrian’s Wall), it’s an excellent boundary delineating “PAX ROMANA” from “HEATHEN OUTERLANDERS,” and it’s a reasonable defence line in itself; random raiding parties would definitely be deterred. Against more determined attacks, it works as a speed bump/tripwire. The troops at the wall fight a delaying action while signalling to the legionary forts. The matchless legions then converge on the incursion and, well, kill everyone.
Pretty much the entire active Roman military would be near a line like this in peacetime, with no strategic reserve; policy forbade having troops close to home, for fear of military coups. With this approach, 300,000 soldiers effectively defended an area the size of Europe, containing a population at times above fifty million, for several hundred years. The decline and fall of the Roman empire occurred for a number of reasons, and the shift in emphasis from frontier defence was one of them.
We now return to your regularly scheduled travelblogs.