The Antonine Wall in Scotland was occupied for a much shorter time than the more famous Hadrian's Wall to its south. Nevertheless it was a substantial line of fortification as this picture of the wall and ditch at Rough Castle suggests.
Large sections of the Roman army were concentrated near areas of political importance, such as the big cities of the east and, of course, Rome, for the praetorian guard and its supporting units steadily grew in size. Some regions within the Empire were never fully under control, with resistance movements or persistent banditry, and also required substantial military garrisons. However, the bulk of the army lay in the frontier provinces. Under the Principate these bases became steadily more permanent and substantial, so that eventually timber and thatch were replaced with stone and tile. Over the last century or so archaeologists have revealed many of the sites, and the distribution of the forts and fortresses seemed to indicate the priorities of the army in each frontier zone. Many units are attested at the same base for long periods, sometimes even for centuries. Alterations in the number and type of units stationed in a province and the location of each garrison have often been interpreted as reflecting changes in the military situation.
As we have seen, the presence of a unit’s administrative headquarters and records at a site actually need not mean that the bulk of its manpower was there for all, or even most, of the time period, or tell us what it was doing. Many units were spread out over a wide area in a number of detachments, performing all sorts of tasks. A fort originally built to house a quingeniary auxiliary cohort was not so specialized in design that it could not be employed for a range of purposes. It might be garrisoned by a unit of the original type, though some or most of its personnel could be elsewhere at any time. Alternatively it might house several vexillations from several units of various types, including legionaries or irregulars. Save for short periods or at times of particular crisis, such a fort is unlikely to have housed substantially more men than the ‘paper’ strength of its original garrison without serious overcrowding. All this was even more true of the much larger legionary fortresses. For the entire legion to be physically present in its fortress at any one time can only have been an exceptionally rare occurrence, once these had ceased to be winter quarters where the unit rested in the months between campaigning seasons. It made no sense to keep bodies of 5,000 or so men waiting idly at their base until needed to fight a large-scale war, when there were so many calls upon the army’s manpower. Instead, legionaries were detached as administrators, builders, engineers, policemen, craftsmen, and to garrison outposts or to serve as entire cohorts or vexillations of other sizes for actual campaigning. Legions provided so many specialists of various sorts that the decision to move one from a province, rather than simply send a strong vexillation to wherever they were needed, was not taken lightly.
A distance slab from Hutcheson Hill on the Antonine Wall marking the completion of a section of the line. In the centre Victory crowns the eagle standard of Legio XX Valeria Victrix. On either side kneel submissive captives with their hands tied behind their backs.
The Roman army was dynamic and busy, its manpower performing a great range of roles, both military and civil. Knowledge of the locations of forts and fortresses in a given period does not in itself tells us what the army of a province was doing on a day-to-day basis. Even so, whilst some troops were posted long distances away from their parent unit, the location of so many military bases in frontier zones does suggest that the main focus of the army’s attention was often in these regions. Considerable expense had been invested in the construction of these bases, and even more in some of the massive frontier barriers such as Hadrian’s Wall and the Antonine Wall in northern Britain. Our literary sources tell us little of activities on the Empire’s frontiers, especially from the late 1st century ad onwards. We are therefore faced with the task of understanding how the frontiers functioned primarily from the archaeological evidence, the few fragments preserved in the literature and logical deduction.
Strategy and grand strategy
A surprisingly fierce scholarly debate continues to rage over the precise nature of the Roman Empire’s frontiers. One view is to see Rome’s overall aim as defensive. The army was stationed on the frontiers to counter the many external threats posed by neighbouring peoples, and so preserve the Roman Peace (Pax Romana) which allowed the settled and civilized provinces of the Empire to prosper. It is a view apparently supported by some ancient sources, for instance the Greek orator Aelius Aristides, who in the 2nd century ad compared the army to a wall running around and guarding the civilized (and of course Roman) world. As early as Augustus’ reign, the geographer Strabo had argued that the Romans had already conquered the best parts of the earth and that further expansion was unlikely to be either worthwhile or profitable. After ad 14 Roman expansion was far less concerted and the conquest of new provinces comparatively rare, which appears to support the idea that emperors now thought more in terms of defence and consolidation than expansion. If defence was the army’s primary role under the Principate, then it was a task which it performed exceptionally well in the 1st and 2nd centuries ad, but far less successfully in the 3rd century when the frontier was frequently penetrated by invaders. Scholars have attempted to reconstruct the system which underlay both the long period of effective defence and the factors which caused the subsequent failures. The sites along each stretch of frontier have been assessed under the assumption that a logical and, on the whole, efficient strategy lay behind them. At the wider level of the entire Empire - where we would talk of grand strategy since the entire resources of the state were marshalled for its long-term benefit - the emperor and his advisors carefully planned where and in what ways to commit their forces, balancing the needs of each region. In many ways this is to view the Roman Empire as a very ‘modern’ state, its main objective to defend its territory and possessions against aggressors, which is the ultimate objective of modern democracies. Rome’s apparent success has been interpreted as providing lessons for the modern world.
The opposite argument tends to emphasize the ‘primitive’ qualities of the Empire, lacking as it did rapid communications and an extensive bureaucracy. The Roman world was substantially a world without maps, and the ancient view of geography was simplistic and crude, making detailed planning extremely difficult. As such, it is argued that emperors could not have implemented any grand strategy, even if they had been capable of devising one. Instead their decisions were almost invariably reactions to an event rather than part of a grand plan. At the lower strategic level on each frontier, the location of military bases was haphazard rather than the product of a system. Furthermore it is argued that there is little evidence for the existence of a concerted external threat in many areas. Both the Parthians, and the Sassanid Persians who succeeded them, rarely provoked a conflict with Rome and were too weak internally to seize and hold significant areas within her eastern provinces. Elsewhere the tribal peoples were too disunited to pose a serious military threat, except very rarely in the 4th century and slightly more often in the 5th century. It is also claimed that, far from adopting a defensive posture, Roman emperors until Late Antiquity still hoped for further expansion, and ultimately to fulfil the propaganda boast of limitless Empire/power (imperium sine fine). Rome remained aggressive, and in many areas the army was stationed in expectation of further conquest.
As yet no widely accepted consensus has emerged, but it is probably best to acknowledge that both interpretations have made some important points. The ancient sources suggest that there was no clear defensive or offensive ethos, but a range of opinions. More importantly they make it clear that the Romans were more concerned with power than the physical occupation of land, and dealt with political entities, states, kingdoms and tribes, rather than simply areas of territory. The Roman Empire extended as far as the Romans were able to make peoples do whatever they desired or, perhaps more accurately, deter them from doing anything which the Romans did not want them to do. The Roman ideology emphasized the need to maintain and protect Rome’s power and reputation. Military defeat or perceived weakness damaged this and required vengeance. Roman campaigns were frequently aggressive - if only because the army was simply more effective whenever it adopted the offensive than when it tried passively to defend - but a successful war did not always require permanent occupation of new territory. Rome was far more powerful than any other nation with which she came into contact. The frontiers occupied by the Romans certainly were intended to oppose attacks from outside, but were not set as limits to Roman power. Rome could and did operate well in advance of these whenever she chose. The reality was that large sections of the Roman army remained based in the same regions on the edge of the Empire for long periods of time, and engaged in warfare with the peoples outside the province. Some central planning did occur, if only in such matters as how many units, and especially legions, were stationed in each province, but whether or not by modern standards this constituted grand strategy is questionable.
Frontier zones
The Romans did not really have a word equivalent to our ‘frontier’. Limes, a Latin word which used to be understood in this sense, largely kept its real meaning of road. Roads were fundamental to any long-term military deployment, connecting individual army bases. Good, all-weather roads made possible rapid and efficient movement of troops and supplies. As stated earlier, Roman forts and fortresses were not primarily defensive structures in the sense that it was rare for their garrisons to fight from behind their walls. The army was usually better trained and disciplined, and had a more sophisticated command structure, than its opponents and was often also better equipped. This gave considerable advantages in open fighting, which could and did compensate for much smaller numbers. Therefore when attacked, Roman units left their fortifications at every opportunity to fight the enemy in the open. The road system facilitated the concentration of units to form a field force.
Several frontier zones were based around significant geographical features, if only because these had often determined pre-existing political geography. Much of the frontier in Africa was formed when the Roman province reached desert areas which were sparsely populated and difficult to cross. The Rhine and Danube in Europe, and Euphrates in Syria, formed important parts of the frontier system in each area. Rivers were patrolled by Roman vessels and transport of men and material by water was often more rapid than overland. They also presented barriers to an invader, especially since the army took care to guard any bridges. A large army would be delayed by a river, since it would need to carry with it, make or gather many boats to ferry itself across. Any delay gave the Romans more time to discover the incursion and gather a force to meet it. Only freak weather conditions, such as the occasional freezing of a river in winter, changed this. Yet these same rivers did not form significant obstacles to the Romans. They controlled the bridges and crossing places, and their navy was active on the water itself. A Roman army faced little inconvenience whenever it wished to advance suddenly and rapidly across a river line against the enemy. Frontier lines were essentially solid bases from which the army could launch an offensive or counter-offensive whenever it desired, not barriers to hinder movement in both directions.
Auxiliary forts tend to be arranged in a line on or near the road running along the fringe of the province. Positions of obvious importance, for instance mountain passes, water sources in desert areas, and river crossings, were usually protected by a fort. There was also a range of smaller military bases, from forts and fortlets for small vexillations to small turrets (turres and burgi) manned by no more than a handful of soldiers. The turrets were a feature along many Roman frontiers, as well as along important roads, and are depicted on Trajan’s Column. In some cases they appear to have been simply observation posts, providing raised viewing platforms and also contributing to the army’s visible presence to surrounding land. Such lines of towers may well have helped to suggest that the army was watching what occurred. In some cases the towers were part of a system for signalling simple messages, using either fire signals or basic forms of semaphore. Observation from fixed points provided the army with some information, but far more could be derived from patrols. One reason for the high proportion of cohortes equitatae in relation to purely infantry units was that the small, balanced force of foot soldiers and cavalrymen were especially well suited to the patrolling and escort duties required of frontier garrisons. In desert areas some mixed cohorts acquired a small number of camel riders (dromedarii), who were especially suited for long-distance patrolling in the arid conditions. At least as important as anything that parties of soldiers themselves saw or heard was the diplomatic activity which went on well beyond the military zone. Frequent embassies to and from Parthia and later Persia are mentioned, but we also read of centurions sitting in on the gatherings of tribal chieftains in late 2nd-century Germany. Friendly leaders often received subsidies, and sometimes military advice or actual aid.
Small watchtowers were a common feature of many frontier systems and also within some provinces, for instance running along important roads. The garrisons of such stations consisted of only a handful of men, who were clearly not expected to deal with major attacks In some cases such towers formed part of a system of beacons or other signalling devices to convey simple messages quickly. This reconstruction of such a tower from the German Limes is based on excavation and the depiction of such outposts on Trajan's Column.
Most frontiers did not have a continuous boundary such as Hadrians Wall. This map of the forts and fortresses along the frontier in Germany at the end of the 1st century AD provides an example of more typical frontier systems. Only a few stretches were provided with linear fortifications
Linear boundaries
The most spectacular examples of Roman frontier systems were the great boundary walls and ditches constructed in northern Britain, Upper Germany, Raetia, and several places in the North African provinces. The use of sizeable linear obstacles was occasionally a feature of Roman campaigns. Caesar constructed such a line to block the passage across the Rhone of the migrating Helvetii in 58 BC, whilst in 71 вс Crassus had tried to hem Spartacus’ slave army into the toe of Italy in a similar way. Sadly none of the more permanent frontier walls receive much attention in our sources. Most appear to date to the 2nd century ad. Yet even though the huge effort required to construct such structures may suggest a growing realisation that the army was unlikely to move forwards again, this is probably mistaken. Hadrian’s Wall was virtually abandoned within a few decades of its building, when the army moved further north to construct and occupy the Antonine Wall. This in turn was swiftly abandoned, and the more southern line re-occupied. The army may then have moved north to re-occupy the Antonine Wall but, if so, this was soon permanently abandoned. Hadrian’s Wall then continued in use for several centuries, although it is possible that this might have changed had the Emperor Septimius Severus not died before the completion of his operations against the Caledonians. Either the current Roman thinking on how best to deal with the military ‘problem’ in northern Britain, leading to different policies, or the situation itself had changed and required different solutions. The ditch and wall lines in Numidia may in turn have been solutions, or attempted solutions, to immediate local problems at specific points in time.
Hadrian's Wall
Rather than attempting to describe each of the different linear boundaries, it is probably best to concentrate on the best known and preserved of them all in a little more detail. Hadrian succeeded to the throne on the death of Trajan in 117, but the circumstances of his adoption as heir were somewhat suspicious and his position was initially insecure. Abandoning most of his predecessor’s eastern conquests, Hadrian spent much of his reign touring the provinces, taking particular care to inspect the provincial armies and ensure their loyalty. It is possible that there was an outbreak of serious fighting in northern Britain at the beginning of his reign. The Emperor visited the island in 122 and ordered the construction of a great wall, according to his 4th-century biographer, to ‘separate the barbarians from the Romans’. Although influenced by the line of the existing ‘Stanegate Frontier’, a line of forts and outposts running along an east-west road, Hadrian’s Wall lay in general a little further north, running along the higher ground, especially in such sections as the craggy Whin Sill. Wherever possible, the wall ran along the crest of this and other ridges, stretching from one horizon to another. There is some evidence to suggest that the wall itself was originally whitewashed, which can only have increased its visible presence.
Hadrian’s Wall was 80 Roman miles in length (73 modern miles or 117 km), extending from Bowness-on-Solway (Maia) in the west to Wallsend (Segedunum) in the east. Over half of the wall was built in stone, but the westernmost section of some 31 Roman miles was originally a turf- and-timber rampart. This section was later replaced in stone, usually on the same line, although in one or two places this was altered, allowing us to see short stretches of the original turf wall, for instance to the west of Birdoswald. This was just one of several major design changes made throughout the wall’s life. At first the stone wall was planned to be 10 Roman feet wide (c. 3 m) and some stretches - known to archaeologists as the ‘Broad Wall’ - were completed to this width. Elsewhere, only the foundations were laid to this size and the actual wall was narrower at 8 or even 6 Roman feet (1.8 m). In at least one place the foundations of the ‘Broad Wall’ were abandoned altogether and a narrower wall built near the top of the crags. The stone wall always had a cobble stone foundation - as did some stretches of the turf wall - an inner core of rubble and outer faces of stone set in lime mortar. Its original height is unknown, nor is it certain whether or not it was simply a barrier wall as in Hadrian’s German frontier system, or was topped with a walkway and battlements along which patrols could move.
The ‘Broad Wall’ marked the original plan, and it was at this stage that the buildings originally intended to form part of the system were marked out and possibly constructed. Every Roman mile there was a small fortlet, known today as a milecastle and conventionally numbered from east to west. Between each milecastle were two small turrets. In several cases milecastles and turrets were clearly built with short sections of ‘Broad Wall’ either side, long before the main connecting wall was completed to the narrower gauge. Milecastles tend to average some 18 sq. m (60 sq. ft) internally, but as already mentioned, each of the three legions constructing the wall followed a slightly different design. The milecastles of Legio II Augusta tended to be wider than they were deep, the opposite of those built by the other two legions. All milecastles had a gateway in their southern wall and another leading north. The latter was topped by a tower and this may also sometimes have been the case with the southern gateway. The survival of stone steps in the milecastle 48 at Poltross Burn demonstrates that the walls within a milecastle did have walkways, and suggests a height of about 4 m (13 ft).
(Above) Hadrian’s Wall is probably the most famous of all the fortifications built by the Roman army. It was built and developed over a long period of time, leading to numerous changes in its design. The function of the wall continues to be hotly debated.
(Above) The milecastles on Hadrian's Wall were small structures similar to the fortlets and outposts on other frontiers.
(Above) The milecastle at Poltross Burn contains one of the few clear traces of stone steps in the installations along the wall. These make it clear that there was a walkway along the wall inside a castle, although this does not necessarily mean that there was a similar walkway and parapet along the wall itself.
A classic image of Hadrians Wall rolling across the craggy landscape, in this case near Peel Gap to the west of Houses leads
In the original design there were no forts actually on the line of the wall. This was changed before construction was complete and forts were added. At Housesteads this meant that the new wall of the fort was built over a demolished turret.
Milecastles in the turf wall were similar in basic design. Internal buildings seem to have been added after the original construction and tend to be fairly crude. As far as possible, positioning conformed to the original regular plan, with the result that some of these fortlets were in highly unsuitable locations. The northern gateways sometimes opened onto a slope or cliff, whilst the entire milecastle might be set into a steep valley which dramatically reduced visibility from the tower. The same rigid adherence to a plan marked the location of turrets. At Steel Rigg a third, extra tower was added, probably to permit observation into a wide area of dead ground left by the original design.
In the original design, the bulk of the garrisons for the frontier zone were to remain in the existing forts along the Stanegate, a mile or more to the south. However, within a few years this initial conception was altered and forts added to the line of the wall itself. In several cases this involved the demolition of existing structures, for instance at Housesteads where the north wall was built on top of an at least partially completed turret. The forts on the wall added to the number of gateways giving the army access to the north. Eventually there were 15 forts actually on or very close to the wall, and other bases in advance of the line, behind it, and on both flanks. Some of the bases behind the wall had clear supporting functions. The military compound in the town of Corbridge appears to have been a depot with significant storage space and workshop facilities. South Shields (Arbeia) fort south of the mouth of the Tyne was at one stage a massive supply dump containing a large number of granaries. As usual, most forts were surrounded by large civilian canabae, and the impression given by inscriptions is of a very cosmopolitan community. The southernmost boundary of the military zone was marked by the vallum - a modern and somewhat inaccurate term which has nevertheless become conventional. The vallum was a broad, flat- bottomed ditch with a low mound running on either side of it. There were two formal crossing places, both controlled by the army.
The Cumbrian coast to the west contained what was effectively an extension of the system, with forts, milecastles and turrets, but no connecting wall. Most of these sites appear to have been abandoned long before the end of the 2nd century, presumably when the Romans’ perception of the military situation had changed.
How did the wall work?
It is clearly mistaken to imagine vast hordes of Caledonian or later Pictish invaders hurling themselves at Hadrian’s Wall as the Roman defenders manned the battlements to oppose them. Even if the wall did have a walkway, the whole structure was not intended as a fighting platform. It would have presented a large enemy army with a difficult, but not impassable barrier. The wall could be scaled with ladders, but this would be a slow way of getting a large number of men across and obviously not practical for horses. It would be better to capture a gateway, but further progress for animals would be impeded by the vallum unless one of the two crossing places could be taken. Any such delays only gave the Romans more time to assemble a field force and move to intercept the enemy. This was invariably the objective here as on the other frontiers, to bring the enemy to battle and defeat them swiftly and decisively. In most cases the mustering of a substantial tribal army should have been reported before it had a chance to make an attack. As on other frontiers, diplomatic activity will have kept the northern tribes under observation. The outpost forts also had an important role to play. Bewcastle appears to have been built around a long- established native shrine, and may have allowed the army to monitor religious gatherings amongst the tribesmen in that area.
The mustering of large armies would have been a rare occurrence. Raiding, often on a very small scale, is likely to have been far more common. A legal text does refer to the case of a woman condemned to penal servitude, but then captured from Roman north Britain in just such a foray. She was subsequently sold back into the province and bought by a centurion named Cocceius Firmus, quite possibly the same man who set up the altars in Scotland. The Vindolanda tablets pre-date Hadrian’s Wall, but it is worth remembering the small number of wounded soldiers in the hospital, as well as a fragmentary text describing the fighting characteristics of the local ‘little Britons’ (Britunculi), which describes light horsemen. Whilst it cannot have been too difficult for a handful of men to sneak across the wall, they could only do so on foot, and returning with any bulky plunder would have been difficult. Such small-scale raids were extremely unlikely to be reported in our sources, and the distinction between these and violent crime was anyway indistinct.
Military activity was probably almost always on a very small scale, but it is a mistake to view this as utterly different and separate from larger- scale attacks. Many of the peoples of the ancient world, especially the warlike tribal societies, viewed raiding and warfare as normal parts of life. Where they were stronger than their neighbours, they did not need any greater provocation to attack them. Successful raids won men plunder and gave them prestige amongst their own people. A reputation for military might helped to deter those neighbours from attacking. Some tribes, especially amongst the Germans, took pride in the amount of land they could keep unoccupied around their borders, for this demonstrated their ferocity and frightened off potential enemies. To such peoples the Romans would have appeared no different from any other neighbours. If the Romans appeared to be weak then they would be raided. Each successful raid added to this perception of their vulnerability, and so encouraged more frequent and bigger raids. A small party crossing into the Empire and rustling cattle or taking a few captives did not seriously challenge Rome’s authority. However, if this was allowed to happen frequently, then aggression against Rome would escalate. Unchecked, then this could lead to large-scale invasion.
Hadrian’s Wall and the other frontier systems are best seen in this light. They helped to mark out Roman territory to any potential enemy, and contained large, impressive structures as demonstrations of Roman might. Linear boundaries in particular helped the army to regulate movements and trade across the area, and made it difficult, if never impossible, for hostile groups to raid successfully. Diplomatic activity and intelligence gathering monitored events beyond the frontier and ideally gave warning of future danger. Yet ultimately the security of the Empire rested more on Rome’s reputation for military strength and this was best displayed when the army took the field. Roman frontiers were never intended to limit or restrict movements of the army, and always permitted punitive expeditions to attack the enemy whenever it was considered necessary. Every Roman victory added to the aura of overwhelming and irresistible force which was the greatest protection of the Empire. Any defeat, however small, dented this reputation. Left unavenged, then the frontier became liable to more attacks. It was no coincidence that an initial defeat in a frontier zone was often followed by more failures and a period of intensive large- scale campaigning to restore the impression of Roman might.