Walls are as old as the same cities, often a continuum with the past whereas new walls were built on the older and with the same materials. So it is difficult to have evidence of what was built before and what after the Roman age. Sometimes the kind of material and the masonry type (opus) helps in finding the solution. Castra (plural of the latin term castrum),  are barracks which are often nearby cities (as Castrum Praetorium in Rome or Castra Albana in Albano Laziale). Valla (lat. sing. vallum) were long defensive lines by the external empire boundary (limes) which could have or not a part in masonry (sometime it came in wood), but the basic structure was a slope just before the wall.