I am currently considering the problem of the route taken by Penda of Mercia when he killed the kings of the East Angles in about 635.
So far as I can tell, the Icknield Way was well defended by a series of dykes and indeed, it is possible that at least some of the dykes were built to keep Penda out. Devil’s Dyke alone would have been a formidable obstacle, never mind the problems presented by Fleam Dyke and the others. The series of dykes are banks and ditches, the ditches on the west of the banks, running from the fens across the Icknield Way to the dense forest to the south east.
Of course, Penda might have travelled by sea and the shortest route from the heartland of Mercia would have been north up the Trent to the Humber and then out to sea to the east.
However, I am also speculating about the Fen Causeway. This was built by the Romans and ran from Ermine Street near what is now Peterborough east across the fens to Denver in Norfolk. The course is somewhat obscure, and seems to have been winding in places, following ‘roddons’, or the banks of old waterways, and possibly also adopting stretches of older causeway. Where it still exists, it seems to have been constructed of gravel to form the familiar wide Roman road bed. But I doubt that Penda was able to march his war band straight into the heart of East Anglia, because of the problem of bridges.
Whatever its route, the Fen Causeway must have run across a series of islands with open water in many places. It has been argued that the Romans built timber bridges in this area and they would certainly have struggled to find much stone for building in the fens. (M.C. Bishop, The Secret History of the Roman Roads of Britain, 2014). So would any timber bridges have survived for 200 years? Waterlogged piles, certainly, but the whole structure? And significant causeways?
My guess is that most bridges would have been in a state of decay and that Penda would have faced a fair bit of building work and/or getting wet. Horses swim quite well – better than armed men on the whole – and they may well have had carts that floated. But the logistics of getting hold of enough boats would have been a challenge.
I am reminded of the story of Hereward the Wake, resisting the Normans from the Isle of Ely in the 1070s. The Normans built a causeway, which sank under the combined weight of their horses and armour…