Vallum

Looking south from the Roman Wall (it's just behind the camera) is a ditch that runs most of the length of the wall on the friendly side, if you were a Roman. It has survived for 2000 years although it is not as deep as it was it is still a significant landscape feature.
Experts seem to think that it was used to mark he edge of the military zone. So anyone in the area between the vallum and the wall would have to had a good excuse!