
After Heddon-on-the-Wall, there's not a great deal of Wall to be seen. Most of the stone was robbed away over the course of many centuries to make houses, barns, churches and dry stone walls. However, as you speed along the fabulous B6318, which is, of course, a Roman Road, you will notice a great v-shaped gash of a ditch which runs through the fields to your left. This is the Vallum, a huge defensive ditch which runs on the South side of the Wall. You can see its profile in this picture - as an earthwork, it is very impressive. Do not confuse it with the Wall Ditch, which is not quite as deep, and runs along the North side of the Wall (some stretches of the Ditch can be seen along the B6318 in some of the fields to your right).
The Vallum seems to have been added some time after the Wall was built. The fact that it is on the South side of the Wall seems to imply that barbarian attacks (perhaps from disaffected groups of the Brigante tribe) could come from any direction. Crossings were put in across the Vallum to allow access to milecastles and forts, but there is only one remaining example of this, and we haven't seen it yet! (Watch this space...)
Both Ditch and Vallum would have held entanglements to trap unwary attackers, particularly horsemen: the Ancient World equivalent of barbed wire consisted of sharpened poles and twiggy branches - very nasty if you fell onto them. The soldiers on the Wall would have had the perfect opportunity to shoot or spear you as you attempted to escape: not unlike shooting fish in the proverbial barrel.