The mile-forts and turrets came first. The forts were built to a simple rectangular plan, fifty feet wide and sixty deep, with gateways in the north wall and the south. Inside there were small buildings and an oven, and a staircase leading to a viewing platform. The turrets were even simpler, just thatched towers set on bases twenty feet square, each with a hearth and a shelter for the soldiers. The turrets and forts were intended to be part of the fabric of the Wall itself, so they were built with ‘wings’, stubby extensions on their east and west faces where the curtain wall would be attached.

Such was the speed of construction that in some places the curtain wall was already being built. On a foundation of slabs set in puddled clay, two courses of dressed stone were laid, rows of neat square blocks. Then a core of rubble bonded with clay or mortar was poured in to fill the space between the skin of cut stone. Drains were built under the Wall, and more significant streams were culverted. The completed stone sections were already being plastered and painted white with lime-wash render, and red stripes ran along the line of the wall, picking out the curtain’s courses of stonework.

Brigonius, a stone man himself, marvelled at the pace of the work. It wasn’t just the hugely efficient transport of stone that impressed him but the Romans’ use of concrete. Without a core of concrete the Wall could never have been built so rapidly, or so robustly. Concrete even set hard under water, allowing the construction of firmly founded bridges. If the facing stones were robbed or decayed in the future, the concrete core would stand; the Romans were building for centuries.

The Wall was not meant to be a defensive barrier, in the way that the wall of a fortress was designed to withstand a siege. Its purpose was control. The regularly placed fortified gates would allow the army to control the flow of population through the area - and, indeed, levy tolls from it. The Wall was imposing enough to deter any small-scale raids, but if there were any larger-scale disturbance the legionaries would mass from their bases in the south and ride out through the Wall gates to meet the enemy in set-piece battles in open country. In fact, Brigonius was coming to understand, the Wall was a mere component in a system of deep control, defence and communication that stretched south of here through roads and forts back into the Roman province, and even to the north, where manned outpost forts would be maintained beyond the Wall itself.