In 1072, the fifth year of the Camp of Refuge, it had assumed so formidable an aspect, that William thought it necessary to take vigorous measures against it, more especially as there had been lately a commencement of correspondence with the Danes. The difficulty was to reach it, for the treacherous ground of the fens afforded no firm footing for an army; there was not water enough for boats, no station for archers, no space for a charge of the ponderous knights, amongst the reedy pools. William decided on constructing a causeway, and employed workmen to cut trenches to drain off the water, and raise the bank of stones and turf, under the superintendence of Ivo Taillebois. However, Hereward was on the alert, harassing them perpetually, breaking on them sometimes on one side, sometimes on the other, in such strange, unexpected ways, that at last the viscount came to the conclusion that he must have magic arts to aid him, and persuaded the king to let him send for a witch to work against him by counter spells. Accordingly, she was installed in a wooden tower raised at the end of the part of the causeway which was completed, and the workmen were beginning to advance boldly under her protection, when suddenly smoke and flame came driving upon them. Hereward had set fire to the dry reeds, and, spreading quickly, the flame cut off their retreat, and the unhappy woman perished, with many of the Normans. Again and again were the Norman attacks disconcerted, and all that they could attempt was a blockade, which lasted many months, and might probably have been sustained many more by the hardy warriors, if some of the monks of Ely, growing weary of the privations they endured, had not gone in secret to the king, and offered to show him a way across the Marches, on condition that the wealth of the Abbey was secured. Accordingly, a band of Normans crossed the fens, took the Saxons by surprise, killed a thousand men, and forced the camp. Hereward and his five comrades still fought on, crossed bogs where the enemy did not dare to follow them, and at length escaped into the low lands of Lincoln, where they met with some Saxon fishermen, who were in the habit of supplying a Norman station of soldiers. These Saxons willingly received the warriors into their boats, and hid them under heaps of straw, while they carried their fish as usual to the Normans. While the Normans were in full security, Hereward and his men suddenly attacked them, killed some, put the rest to flight, and seized their horses.