Actual documentary evidence detailing the purpose of Hadrian’s Wall is virtually non-existent. The ‘Scriptores Historae Augustae’ tell us very little, in fact the sole classical literary reference linking Hadrian to the wall was written by Aelius Spartianus, towards the end of the 3rd century. It simply stated: ‘He instituted many reforms and was the first to build a wall eighty miles long to separate the barbarians from the Romans’.
However, there is a considerable amount of epigraphic and circumstantial evidence which tells us how and why the wall was built and, when pieced together, gives us a good indication of its purpose. Hadrian became emperor in 117 AD after the death of Trajan and his political aims were completely different to his predecessor. Whereas Trajan had pursued a policy of uninhibited expansion, Hadrian wanted to consolidate existing gains and build permanent, secure frontiers, thus making a statement of the empire at its greatest and largest.
In general, Hadrian’s objective was defence rather than attack and this led to the construction of fortified boundaries, not just in Britain, but in many parts of the empire. At the beginning of Hadrian’s reign there was obviously trouble in Britain, as indicated by his biographer, who wrote, ‘the Britons could no longer be held under Roman control’, and as most of the southern parts of the province were settled it is likely that the rebellion was in the north.
Although we are given no further details of the trouble, there is evidence that an insurrection was suppressed, by Pompeius Falco governor of Britain from 118 to 122 AD, from an inscription discovered at Jarrow, commemorating the victory in 118 AD and the issue of commemorative coins in 119 AD. Also, at about this time, the fort at Brough on Humber was re-occupied for a short time, which would suggest that the rebellion involved the Brigantes, a tribe within Roman Provincial territory and their allies, the Selgovae and Novantae, from lowland Scotland.
In 121 AD Hadrian began a tour of inspection of the Western provinces and, upon his arrival in Britain, appointed Platorius Nepos as governor in 122 AD, to carry out his policy of establishing a fixed permanent frontier and also brought over the Legio VI Victrix, to assist in the task. The other frontiers of the empire were usually formed by natural boundaries: a sea, or a great river such as the Rhine or Danube, or a desert as in Africa, but in North Britain there was no such clear demarcation line.
Normally in these situations the frontier was defined by a path or military road (Limes), such as the Stanegate, with forts and signal towers. However, man-made constructions were not entirely new, because Domitian had strengthened the Upper German frontier with an actual fence and this had been further developed by Hadrian who constructed a continuous wooden palisade with look-out posts, signal towers and forts close behind the line. Such structures had no real military value, but defined the frontier and supervised small-scale crossings of the border by the use of designated crossing points or gates.
In contrast Hadrian’s Wall was a much stronger barrier constructed in stone, supported by a system of forts and roads connected to the military fortresses of York and Chester. Frere suggests that this type of structure was needed not only to define the frontier and separate ‘Romans from barbarians’, but that at the time it was constructed it also served to divide the war-like Brigantes from the equally unruly Selgovae and Novantae tribes. Archaeological evidence suggests it is likely that theses tribes had previously joined forces against the Romans and, of course, the Romans would have keen to stop this happening again.
The Stanegate frontier ran from the Solway estuary to the Tyne and was just a stone road, so didn’t really stop anything. There was no merit to be gained from advancing up into Scotland, but the Romans were not prepared to give up what had been gained so the best option was to divide the province with a permanent structure. In addition by removing the likelihood of an alliance between these tribes it allowed for an experiment to be carried out in as much that the military supervision of the Brigantes was considerably reduced in the hope that if left to themselves, they may become less rebellious.
This also meant that the main strength of the occupying forces could be used to man the Solway-Tyne isthmus. In the long term the prevention of border skirmishes would have the advantage of providing peaceful conditions for economic development in the region behind the wall. Excavation on the line of the wall shows that it’s original purpose was to strengthen the Stanegate frontier with the addition of a ditch and a wall with mile castles and turrets.
There are three river valleys along the route of the Stanegate, the Eden, Irthing and Tyne and the wall was on the northern crest of these valleys so that the rivers were in Roman territory and there was a good outlook to the north. The wall itself was ten feet thick and, including the parapet walk, twenty feet high. From Newcastle to the crossing of the Irthing it was made of stone and west of the Irthing of turf because there were no convenient limestone supplies for the mortar.
At every Roman mile there was a fortlet for the patrolling garrison, containing one or two small barracks for accommodation for eight or thirty men. Massive, wide gateways were set into the walls so that troops could be rapidly deployed from behind the screen offered by the wall itself and the gates also served as a means of controlling and scrutinising any civilian traffic passing through. Equidistant between the mile castles were two turrets, or stone towers, and their purpose was clearly for observation and signalling.
When construction had reached the north Tyne the specification was changed and forts were added to the wall itself for the purpose of housing the fighting garrisons that had previously been left on the Stanegate to the rear. Archaeologists also discovered that the wall was extended by four miles at its east end, taking it to Wallsend, and giving its total length of eighty miles. The reason for this extension was probably to take the wall to a point where the widening estuary of the Tyne alone would be sufficient to prevent infiltration across the boundary.
A further modification was made West of the Irthing where five miles of turf wall was replaced by stone and this was possibly related to a change of the garrison at Birdoswald. It appears that the Birdosland fort was built to house a cavalry regiment and was projected through the wall with three gates to the north for rapid deployment, but as it was actually garrisoned by infantry, these gates were not required, so the opportunity was taken to provide more space around the fort by diverting the wall to join the fort’s northern corners.
Under the new specification the wall was only eight feet thick and this may have been influenced by the decision to move the garrisons up to the wall, which offered greater security, and reducing the size would save money and materials. . There is evidence that the forts were a secondary consideration to the wall system, shown by the fact that at Chesters, Housesteads and Great Chesters, mile castles or turrets attached to the broad foundation, underlie the forts.
And at Birdoswald the turf wall, with its ditch and a turret, underlie the fort. However, the decision to move the garrison up to wall must have been taken early in its construction because the forts at Halton and Benwell produced inscriptions of Platorius Nepos, who is thought to have left Britain in 125 AD. Further evidence that the purpose of the wall was to be a secure, permanent frontier comes from the fact that once the garrisons were moved up to the wall, the Stanegate forts were evacuated.
Also there had only been seven forts on the Stanegate frontier, whereas the final number of forts on the wall totalled sixteen and garrisoned in excess of 10,000 men. Once the position of the forts had been fixed, the Vallum was constructed and although a great deal is known about its construction, it’s actual purpose has been the subject of various theories. The Vallum was a formidable ditch, twenty feet wide at the top, ten feet deep and with a flat bottom eight feet wide.
On each side there was a shoulder thirty feet wide and turf mounds twenty feet wide and six feet high. The purpose of this construction appears to be a boundary that was designed to be impassable, or at least, very difficult to pass on foot. The only way across it was through big stone gateways at the forts and at one or two of the mile castles. Originally it was thought that this was the Hadrianic frontier, but the fact that it follows the wall from end to end and deviates from it’s course to avoid the sites of forts, proves that it was planned later.
At the forts, the causeways were faced with stone and the gateways were controlled from the north. Whereas at the miles castles there were gaps in the north mound, but not in the south and the causeway was lightly metalled, which suggests that it formed a track which could be patrolled and a way that supplies could reach the mile castles. Until the military road was constructed, later in the century, this trackway was probably the only means of supplying the mile castles.
R. G. Collingwood’s theory was that the Vallum was a civil frontier manned by the procurator’s staff as a means of raising taxes, but it’s close relationship to the forts and the extraordinary effort involved to make it continuous, clearly suggests that it belonged to the military. Nothing was allowed to stop its course: where hard rock was encountered it was chiselled out and at White Moss, where the ground was too waterlogged to cut a ditch, it was built up like a canal. With all this trouble and expense being taken to make it continuous it was obviously an important feature.
However, its purpose could not have been a strictly military defensive system because it takes no account of commanding ground. It is much more likely that its purpose was to act as a barrier and a line of demarcation, which effectively prevented entry except at fixed points, and specifically made any attempt at contact between the Brigantes and their allies in the north very much more difficult. The fact that this elaborate structure was necessary suggests that there was large scale unrest to the rear of the wall and moving the fighting garrisons to the wall itself shows that there was also intense hostile pressure from the north.
Archaeological field-work and aerial photography are now beginning to show the purpose of the wall was to divide these rebellious factions. To the east the wall coincides approximately with the boundary of Brigantia, but in the west cuts across Brigantian territory to make contact with the southern shore of the Solway. The eastern end of the wall was protected by the deep estuary of the Tyne, and a fort at South Shields overlooking the mouth of the river and the configuration of the coast prevented any threat.
However, in the west the position was not as commanding and consequently four outpost forts were built north of the wall for the purpose of providing early warning of hostile movements and also to maintain Roman control over the outlying parts of Brigantia. The forts were at Birrens, Broomholm, Netherby and Bewcastle and all have produced Hadrianic inscriptions which imply that they were in existence by 128 AD, although the Bewcastle example is too fragmented to be reliable.
In addition, the fortifications were extended down the coast with mile castles and signal turrets, which were almost certainly for protection against sea-born landings. Hadrian’s wall was obviously not a simple structure and turned out to be much more complicated than had at first been planned. It obviously took a considerable time to complete and although the main features had begun to appear before the end of Platorius Nepos’ governorship, the coastal system and the fort at Great Chesters was still being built in or after 128 AD.
The fort at Carrawburgh revealed an inscription of Julius Severus, governor from 130 -133 AD and at Carvoran fort there is an even later reference to Aelius Caesar, Hadrian’s heir, who took this title in 136 AD, so this is likely the work of the governor Mummius Sisennia. This tells that the complete system took in excess of ten years to build. The system itself was certainly not designed as a fighting platform because the garrisons were not equipped, or trained for this sort of warfare.
This would be completely contrary to the tactics of the Roman army who preferred the open terrain, no evidence of any special weaponry has been found and in any case the wall would have been to narrow. If there had been any major threats the Roman army would have dealt with it in advance, trying to meet the enemy in the open territories. The intention of the wall was almost certainly political rather than primarily military and was there to control movement and provide efficient observation of the surrounding area.
Its size was dictated by the strength and hostility of the populations on both sides which demanded a much more powerful structure than had been necessary elsewhere. Although the purpose of the wall was to control movement, not prevent it, it almost certainly caused resentment by interfering with traditional movement from pasturage to pasturage and loss of freedom, but undoubtedly served its purpose of giving early warning and getting troops to trouble spots as quickly as possible.
Hadrian’s wall was not just a simple barrier, it was a real frontier system with six elements that marked the extent of the Roman province. First of all there was the wall itself, the most obvious part of the system. Secondly just to the north of the wall was a ditch, making the approach difficult. The mile castles and turrets spaced evenly along the wall provided gateways and an observation system. Forts attached to the wall every seven to eight miles to ensure a reasonable military presence for its control.
Causeways provided access for supplying the mile castles and later the military way ensured communication along the line of the wall and finally the Vallum restricting access to the frontier from the rear. The wall was an artificial frontier: the finest Roman artificial frontier in its elaboration and in the impressiveness of its remains, and although no unanimously agreed set of reasons exist as to why it was built, by building it, Hadrian had created an enduring monument to himself and to the rest of the world.