The Antonine Wall is the northernmost boundary of the Roman Empire. An exercise in military engineering nearly 2,000km from Rome, it was designed and built explicitly as an example of the might of Emperor Antoninus Pius. Though half the length of Hadrian’s Wall, its southern ancestor, it deserves recognition not only as a high water mark of the Empire, but as a marvel of engineering in its own right.
Today’s military engineers receive their training largely through university degrees, often at military-focused schools such as Norwich University’s school of civil engineering. Not so the Roman military engineer, part of the legion’s immunes class. Raised from the munifex classes (rank and file soldiers), the engineers who built Rome’s far-flung network of roads, fortifications, and many of its more distant cities (a means to keep them busy and productive during times of peace) learned on the job.
For the Roman engineers who built the Antonine Wall, their on-the-job training involved a gargantuan project. The wall was far more than simply a pile of rocks or a mound of dirt. The wall was a complex made of numerous elements. These all played together to create a substantial networked defense, one sufficient to provide Rome some of its last victories before it began the slow decay that would culminate in the departure of the Empire from Britain some 200 years later.
The most obvious and important structure in the Antonine Wall is, of course, the wall itself. The wall may have originally been intended to be a stone wall, similar to Hadrian’s Wall. If so, this plan changed before any significant work took place, as only the foundation of the wall was made of stone. The rest of the wall was an earthen work structure.
The stone foundation of the wall was five metres across and roughly half a metre deep. The stone was more or less jumbled rather than being fitted stone, with regularly spaced culverts along its length. This allowed for quick and easy drainage of water at the base of the wall, helping reduce the odds of the wall becoming waterlogged and collapsing or eroding away.
On top of this base the wall was built of earthen materials. For three-quarters of the wall’s 63 km length the principle material for this was turf. The roots spread throughout this soil made it inherently cohesive, allowing a very steep structure to be formed. The remaining sections of the wall were made of less suitable clay and soil which has not withstood the test of time.
The wall rose three metres into the air with a 70° slope on both faces. This left a flat top not quite two metres wide. Though no accounts confirm this, it is likely that this would have held a wooden walkway and solid wood fence to protect soldiers manning it.
Beyond the Wall
North of the wall stretched a narrow space of relatively flat land (depending on local terrain). This would be riddled with deep pits, forcing attackers to slow down and weave around, breaking up the momentum of any assault. These pits ranged in size from only large enough to capture and snap a leg to being large enough to swallow several people.
Forming an outer boundary to this dangerous terrain in most places was a very deep ditch or moat. While there are some indications that the ditch could be flooded at need, in most cases it likely would have remained dry. Dry or wet, this ditch would have been a substantial impediment. The exact distance between the wall and the ditch varied, as did the width and depth of the ditch itself. But on average the ditch was 12 metres wide and four metres deep.
The detritus created by the digging of the ditch was flung out to the north. This created a low berm that was insufficient for attackers to hide from Roman missiles behind, but which still would have required an assault to climb up. It also artificially increased the depth of the ditch.
The landscape itself formed a part of this defensive structure. The Antonine Wall was built on the south slope of the valley formed by the River Carron and the River Kelvin. The land around these two rivers was substantially boggy, inhibiting the movement of any force preparing an attack upon the wall.
Behind the Wall
Immediately behind the Antonine Wall, the Roman military engineers built a series of fortified camps. The most important of these camps, spaced 3.3km apart, were substantial in size. They contained administrative offices, barracks, warehouses, armories, bathhouses and all the other necessities for a garrison. In some cases, civilian towns would have formed alongside of these forts, housing civilians who had either married members of the garrison or who were engaged in trade in the region and took advantage of the safety provided by the Roman presence along the wall.
Smaller fortlets were also built along the wall. These lacked amenities, but provided sufficient housing to allow small garrisons immediate access to every part of the wall, and an immediate defense to be mounted long enough for reinforcements to arrive.
Finally, the Roman engineers created a military way behind the wall. This Roman road was built explicitly as a means to move troops and supplies behind the safety of the wall. In most areas this road consisted of a layer of gravel and small stones over a bed of larger stone. In more heavily trafficked zones, cobblestones would replace the gravel. Regardless of the surface material used, a slight slope drained water away from the road, keeping it from becoming flooded or muddy. All trees and bushes between the wall and an area a considerable distance south of the way were cleared to ensure that ambush was impossible.
The Mathematics of the Antonine Wall
The sheer volume of both manpower and materials the Roman military engineers had to move and use to construct this northernmost bulwark of the empire is staggering to consider. The Antonine Wall is 63km long, running from the Firth of Clyde to the Firth of Forth. Behind the wall they built 19 major forts and numerous smaller fortlets. The military way stretches the entire length of the wall.
In order to accomplish this, the Romans employed three legions, the II Augusta, VI Victrix, and the XX Valeria Victrix. The typical legion was nominally 5,000 men strong. This means that as many as 15,000 men may have been available to construct the wall, although most estimates state that at most 7,000 would have worked on the wall at any given time. Work would have taken place largely during the summer, when the weather was sufficient to permit construction. The work began in AD142 and ended in AD154. At a rough estimate this means the engineers would have used somewhere between 100M and 200M man-hours of labour.
During this labour, the legionaries employed to build the wall would have moved more than 1B kilos of stone in order to form both the 4.5m wide base of the wall and the military way that runs behind it. Another 1B kilos of turf, clay and dirt were shifted to dig the ditch and build both berm and wall. 252km of lumber was needed to form the walkway and wall along the top which required more than 3,000 trees to be felled.
Once this massive effort was completed, the Antonine Wall was garrisoned by around 6-7,000 men, mostly Roman auxiliaries. The wall was abandoned after less than two decades, though it would be re-occupied for another three years around AD208. In spite of its short tenure it clearly did serve its purpose. At least three of the forts along the wall show clear evidence of having come under heavy attack, probably during the Pict attacks of AD158, four years before to the withdrawal back to Hadrian’s Wall.
Jim Hinton is a combat vet and armchair historian. You can follow him @jamiemhinton