William was encamped with his army at Brandon, but could not find a way through the fens & finding that he could not make a successful attack upon Ely from the direction of Brandon, William moved his army to Cambridge, and tried to gain access to the Isle of Ely across causeways at Stuntney, Little Thetford and Aldreth...

He was prevented from doing so by Hereward and his followers. They exploited the remoteness of the Island of Eels...
The Normans would have to prepare wooden rafts to enable a full assault on the settlement. Hereward allowed, even worked on the building of the rafts and bridges only to booby trap them with explosives and weaknesses. Then when the bridges were most full for the attack, Hereward did set them alight...

Many were said to have perished and were taken with fear of the marsh and it`s deadly gumption...