The problem with answering this question is that we don't exactly know the purpose of Hadrian's Wall. The most obvious answer, and certainly the one that holds the public imagination, is a defensive border wall intended to keep the barbarians out of the Roman province. Comparisons with the Great Wall of China certainly support this, but there are some important structural differences with Hadrian's Wall that make it harder to see its purpose as purely defensive.

Firstly, the biggest problem with interpreting Hadrian's Wall is the state of the archaeology. What remains of the structure is rarely more than a metre or two high, and has totally disappeared in many places. This makes it very difficult to reconstruct what the original structure looked like, which in turn makes it hard to say whether its purpose was exclusively defensive. For instance, archaeologists have argued long and hard about whether there was a walk-way on top of the wall or not. In lots of parts of the wall, the remains are not wide enough to make the top of the wall easily defendable, and it is rare that two men could walk side-by-side. Even if it was possible to patrol the top of the wall, we don't exactly know how soliders would have got up there - no staircases survive in any of the milecastles or turrets along the wall.

This brings me to the next problem: manning the wall. In its first phase of construction, there were no forts on the wall. In terms of defensive structures, there was a milecastle every Roman mile and a turret every third of a mile. Some of the milecastles had rooms for men to sleep in, but some could only house around 8 soldiers - nowhere near enough to defend a mile of wall. Even when the forts were brought up to the line of the wall there was no road connecting them, meaning that reinforcing sections under attack would not have been easy.

The milecastles themselves raise another question. There were 80 in total, and every one of them had a gate through the wall. 80 gates seriously compromise the defensive nature of the wall, making it much easier to break through. These milecastles were not strongly defended, and so a determined force could have easily overwhelmed the garrison. There is some indication that the Romans realised this when the wall was reoccupied after the abandonment of the Antonine Wall, as lots of the gates were narrowed or closed entirely.

So, if Hadrian's Wall wasn't purely defensive, what was it for? There are a number of competing theories, and it would take ages to cover them all in detail. One of the more convincing ideas is that it was a way of regulating movement and trade across the border. That might explain all the gates - rather than defensive positions they could have been customs stops, where soldiers acted as inspectors and tax collectors. We know from excavations in modern Scotland that Roman goods were traded north of the wall, and it's likely that the imperial authorities had some way of regulating the movement of goods and people. These leads into another possible fuction of the wall: a display of Roman power and authority. What better way to stamp your control onto an area than by building a huge wall that blocks free movement through it? Don't forget that there was no history of stone architecture in this region before the Romans arrived, so Hadrian's Wall would have been impressive, perhaps even terrifying to the local population. Constructing the wall says to the locals "this is our land now, and we will do with it what we please. Also, we're not going anywhere soon, so get used to it."

A final possible reason for the wall, and perhaps the most cynical. We know that at least 3 legions were involved in its construction - around 15,000 men. Roman emperors knew all too well what was likely to happen if their soldiers got bored restless. Over the first century of the Roman imperial period, a number of mutinies, rebellions and usurpers had been sparked by under-paid, under-utilised legionaries. Can you think of a better way to keep 15,000 soldiers busy (and tired) than having them build an 80 mile long, 5-metre high wall, and then have them patrol it?