Hadrian’s Wall, Vallum or Fosse
Segment VIII
Brocolita to Vercovicivm
Carrawburgh to Housesteads
Mile Castles: 31 - 36
Built: after 128, dating uncertain
What is commonly called the “Vallum” was a wide ditch flanked by earthen ramparts that ran behind Hadrians wall. Its purpose remains uncertain as it is unique to Roman border fortifications. Possibly it was to define the military zone of the wall and to force trade and transport to follow marked roads to gates in the wall. In many places, it is more visible than the wall itself.
See: http://structuralarchaeology.blogspot.com/2010/11/40-reverse-engineering-vallum.html
https://en.wikipedia.org/wiki/Vallum_(Hadrian%27s_Wall)