The First Plan for Hadrian's Wall
The building of Hadrian's Wall was a complicated process, containing several changes of plan. The first scheme was for a wall of stone 10 Roman feet (3m) wide from the River Tyne westwards for 72km to the River Irthing and a turf rampart 20 Roman feet (6m) wide for the western 45km from the Irthing to the Solway Estuary. The builders made good use of local geographical features. The well-known central sector, actually only about 20km long, ran along the crags, but to the east the Wall was placed on a long ridge running eastwards to Newcastle, while to the west the Wall was often located on shorter ridges, all normally giving wide views to the north.
The stone wall was probably planned to be 15 Roman feet (4.4m) high. It had dressed facing stones in a soft mortar and a core of earth or clay with stones. There is some evidence that there was a wall-walk and parapet along its top.
In front of the Wall lay a berm, normally about 20 Roman feet (6m) wide. In places in the eastern 17.5km of the Wall pits have been found on the berm. In one area at least, where there were three rows of pits, each held two substantial posts, perhaps cut down tree trunks with the branches trimmed short and sharpened at their ends. In one sector the pits were recut. Beyond the berm lay a ditch. It ran along the whole length of the Wall, except where the crags or similar features rendered it unnecessary. It was probably planned to be 30 Roman feet (9m) wide. The material from its excavation was tipped out to the north to form a wide, low mound.
At every mile along the wall was placed a gate protected by a small fortlet termed a milecastle. The milecastles were of stone on the stone wall and turf and timber on the turf sector. Most of the excavated milecastles held one building, presumably a barrack-block, probably for about 8 men. Two milecastles appear to have held two double-sized such buildings suggesting a larger garrison. Some milecastles contained hearths and an oven, while in one a staircase leading to the top of the milecastle wall has been found.
Between each pair of milecastles lay two towers, called turrets. These were always built of stone. A platform on the ground floor was presumably the base for a stair or, less likely, a ladder. The primary purpose of the soldiers based in each turret was presumably observation, and therefore there should have been a tower over the north gate of each milecastle. The placing of the door on the ground floor of the turret suggests that security was not an important concern.
The Second Scheme
The first plan for Hadrian's Wall was not completed before a major change was made. At intervals of about 11.2km a fort was placed on the Wall. It would appear that wherever possible the intention was to place the fort astride the Wall with three of its gates north of the linear barrier and one to the south. Moreover, all these gates were, unusually, double portal with the rearward gate supplemented by two single-portal side gates. The effect of this positioning was to improve the mobility of the army in the frontier area. This decision was not taken lightly for it involved the demolition of sectors of the Wall, turrets and even a milecastle already constructed and the in-filling of lengths of ditch, while many forts in Wales and northern Britain were abandoned to provide troops for the new forts on the Wall.
At about the same time another significant change was made, the construction of a great earthwork, known since the time of the Venerable Bede writing in about 730 as the Vallum, behind the Wall from Newcastle to Bowness-on-Solway. It consisted of a central ditch with a mound set back on each side. It formed a formidable obstacle and perhaps should have been seen as the Roman equivalent of barbed-wire protecting the rear of the frontier zone. Now crossing the frontier was only possible at a fort where the access point, a causeway, was surmounted by a gate. The number of points where the frontier complex could be crossed was reduced from about 80 to about 16.
There were further changes to the Wall, but of a different character. The width of the Wall was reduced from 10 Roman feet (3m) to sometimes as little as 6 feet (1.8m). There was also a clear decline in standards of craftsmanship with later structures much more shoddily finished off than earlier ones. It is difficult to know when Hadrian's Wall was completed. An inscription suggests that one fort was not finished until after 128. Nearly ten years later, the fort at Carvoran was being repaired or rebuilt, though this was not a normal Wall fort. Also before the end of Hadrian's reign, a start had been made on replacing turf wall in stone, though by this time only 8km had been built.