The solution adopted by the Romans across northern Britain was a wall, built in stone in the eastern half of the isthmus, but from the river Irthing westwards it was initially built of stacked turf. The stone wall was initially built 10 Roman (3m) feet wide, but after two years of construction the width was reduced to between 6 and 8 Roman feet (2.5m). In places, such as at Planetrees the narrower wall was constructed on the prepared broad foundations.
North of the Wall, except where natural topography made it unnecessary, ran a V-shaped ditch, up to 6m wide and 2 m deep. At intervals of 1 Roman mile were fortlets (known as milecastles) attached to the rear of the Wall, with gateways through their north and south sides. These were approximately 20m square and accommodated possibly 30 soldiers.
Between each milecastle and spaced one third of a Roman mile apart were towers (known as turrets) built into the width of the Wall and projecting south of it. They were probably for observation and possibly signalling. Probably two years into the building of the Wall it was decided to add sixteen forts, mostly attached to the Wall. These provided the main garrison of the Wall, each fort accommodating between five hundred and a thousand auxiliary troops, both cavalry, infantry and mixed units.
Enclosing these military features to the south was a ditch with a flat bottom and virtually perpendicular sides, flanked by turf mounds on both sides, known as the Vallum. This created a zone behind the Wall, between a few metres and almost a kilometre wide, to which entry from both north and south was controlled by the Roman army, with crossing points over the Vallum only at each of the forts on the Wall.
Civil settlements flourished outside the forts, containing shops, workshops, inns, bath houses, temples, and a variety of other buildings to serve the soldiers' needs. The soldiers' pay attracted traders to settle outside the forts and some of these settlements grew to the size of small towns. Beyond these were the cemeteries.
The Wall itself as a continuous barrier stopped at Bowness on Solway, but beyond regularly spaced forts, "milefortlets" and towers extended the defences down the Cumbrian coast, certainly as far as Maryport. Three forts beyond Maryport are known and included within the WHS - Moresby, Burrow Walls and Ravenglass.
Over time a number of changes were made. In the latter half of the 2nd century a large proportion of turrets were demolished, the north gateways of milecastles were reduced in width and a new road, the Military Way, connected the forts, milecastles and turrets. Probably late in the 2nd century the Vallum went out of use.