The Vallum is a massive earthwork constructed shortly after Hadrian’s Wall itself and lying just south of it. Many visitors confuse the Vallum with Hadrian’s Wall itself because it’s such an obvious and impressive feature in the landscape.
Crossing points seem to have been located south of each of the forts along Hadrian’s Wall and near several of the milecastles. Evidence from the excavated Vallum crossing at Benwell in Newcastle shows these crossing points had impressive monumental gateways.
The Vallum’s purpose is unclear. Many archaeologists think it marks the southern boundary of a military zone with the Wall itself forming the northern boundary. This would have helped protect the rear of the Wall and its associated military installations, with civilian access being closely controlled. The gateway at Benwell supports this idea. The numerous gateways along the Wall at forts and milecastles suggest that the frontier was intended as much to control movement as to provide a defensive line. Traders would have moved goods across the frontier but their movements would have been controlled and their goods taxed.
Relatively soon after it was constructed, some 20 to 30 years perhaps, the Vallum seems to have lost its function – the mounds were cut through and the ditch filled in at fairly regular intervals. It was out of use by the time the forts along the Wall were re-commissioned in the late second century AD following the return of the garrison from the Antonine Wall.