Wansdyke is a large bank and ditch earthwork that extends (in fragments) from just south of Bath to the Savernake Forest south of Marlborough. You can find remnants of Wansdyke along a stretch of almost 40 miles. The Wansdyke dyke runs from the Bath area for 13 miles, then disappears for 15 before continuing on towards Marlborough for another 10 miles. East and West Wansdyke were probably originally one large work, though some suggest that they may have been two separate dykes.
The Wansdyke dyke could have been either a defensive work (to protect from attack coming from the north) or as a line of demarcation between territories. Such a large fortification would have required a large, well organised workforce to build it and a large army to patrol it. It may have marked the northern boundary of Dumnonia, a kingdom, covering from Wiltshire to Cornwall.
The Wansdyke dyke is generally accepted as post-Roman and pre-Saxon, which fits the Arthurian time frame. Post-Roman because because it is cruder than Roman constructions and also the Romans would have had no need for such a boundary in a land they had no trouble controling.
The name Wansdyke in Saxon means Woden's Dyke, which helps in dating, as the Saxons were converted to Christianity in the 630s, so the implication is that Wansdyke was built before that.
Southern England Arthurian Sites