Cambridge (Grontabricc in the 2nd edition Ordnance Survey map of Britain in the Dark Ages extract below) is straddled by 3 large, ancient, earthworks. To the South the Heydon Ditch, to the East the Fleam Dyke and to the North the Devils Dike. They are part of a series of boundary ditches marking the limits of the Saxon Kingdom of East Anglia. They cut across the Roman roads (now designated A505, A11) from the South into the kingdom of the North and South folk of the East Engle.
The exact function of the dykes is uncertain but their Anglo-Saxon origins are agreed:
The four dykes which span the open stretch of country between the East Anglian Heights and the Fenland in the neighbourhood of Royston, Cambridge and Newmarket have been the subject of some controversy. Archeological evidence suggests that they are all post-Roman in date. It has been asserted that they are relics of a fluctuating frontier between Mercia and East Anglia, but the known facts of 7th Cent history do not support this. They may be the relic of a post Mons-Badonicus 6th Cent phase in which Anglo Saxons in temporary retreat secured themselves in East Anglia against British pressure. However this may be, they inspire considerable respect for the powers of their constructors. Less important, but perhaps belonging to the same early phase in East Anglian history, are the short lengths of dyke in West Norfolk.
From the Introduction to the 2nd Edition of the Ordnance Survey Map of Britain in the Dark Ages 1965
Both the Dykes cut across both the Roman road and the ancient Icknield Way from the South, the Roman Road is still the principal route into Norfolk from the South: the A11.
Christopher Saxton (1576) shows the Fleam Dyke and 2 lines running SE from Reach. The line from Reach to Dullingham follows the Swaffham Bulbeck Lode (or possibly a minor Roman Road in use at the time; he shows some of these elsewhere). The line through Stechworth is the Devils Dyke. I have emphasised Saxton’s lines, as they are very faint. Saxton shows nothing of the Heydon Ditch suggesting it was an indistinct feature then, as now.
The expansion of the Kingdom by the marriage of Ethelreda, daughter of King Anna, to Tondbhert king of the Gyrdwas in 652AD rendered the Dykes redundant by pushing the border West to include the wetlands of the fen.
The Saxon kingdoms of England showing ‘East Englas’ comprised the 3 crowns of Norfolk, Suffolk and the Isle of Ely (now, in effect, Cambridgeshire). The kingdoms are as described in the 9 versions of Anglo Saxon Chronicle written between 890 and 1156. The boundaries largely persist as the modern county boundaries to this day. Yorkshire was yet to emerge as an entity being a mere district of the North Humberlands at the time.
The determination to block routes into the kingdom is impressive. It has been suggested (Julian Richards BBC4) that the Saxon invasion of Britain was less of a force of arms (as was thought in 1965) than an entrepreneurial ‘take over’ by trading pioneers who exploited their links with the mainland of Europe to establish familial dynasties that ultimately lead to the creation, under Alfred, of the Burgh system of market distribution and governance, leading to the unification of the kingdoms of England.