Vallum

The Vallum was part of the Roman legionary camp defences usually consisting of a packed earth rampart topped by a palisade.
The Vallum also refers to a massive earthwork which runs parallel to Hadrian's Wall and along its entire length of 74 miles.

The Vallum appears to have been built at the same time as the Wall (122-128 CE).
It runs in a dead straight section before turning and running in another dead straight section, similar to a Roman Road.
The Vallum is located behind the Wall and consists of two embankments on either side of a very wide Ditch which are thought to have protected the Wall and its Forts from attack from behind. The Ditch may have been filled with anti-infantry devices such as the Caltrop.
Positioned between the Vallum and the Wall were the Forts at set intervals and the Military road which often runs on top of the north embankment.
The Vallum runs close to the Wall, and only when the Wall runs on top of the crags of the Great Whin Sill does the Vallum lie in the valley below.
The only other Roman Frontier to have a similar structure is in Tunisia.
The Ditch is only interrupted by causeways placed opposite the gates of the larger Forts.
The Vallum would appear to have only been in use for 10 years. Later, parts of the ditch were filled in at regular 134 feet (41m) intervals and the ramparts cut away.

The total width of the Vallum was 100 feet (30m).
It consisted of a flat ditch 20 feet (6m) wide and 10 ft (3m) deep.
The soil from the ditch was used to create the embankments, which are set 30 feet (9m) away on each side of the ditch and are 20 feet (6m) wide and 7 feet (2m) high.
At White Moss, the Ditch is not dug out, but created by mounds on either side.
For much of the distance, a third smaller embankment was inserted between the edge of the ditch and the southern embankment.

The Fossatum Africae was a fortified Roman frontier in North Africa, stretching 460 miles (750 km), and defending the Roman Province of Africa Proconsularis.
It was possibly started in the reign of Hadrian as it had a similar construction to Hadrian's Wall in Britannia.
The Fossatum consisted of a ditch 10-20 feet (3-6m) in width, with embankments on both sides, similar to the Vallum behind Hadrian's Wall.

The Vallum at Carrawburgh, Hadrian’s Wall