of Gloucester marched east, attacking Stephen's rearguard forces at Wallingford and threatening an advance on London. Stephen was forced to give up his western campaign, returning east to stabilise the situation and protect his capital. At the start of 1140, Nigel, Bishop of Ely, whose castles Stephen had confiscated the previous year, rebelled against Stephen as well. Nigel hoped to seize East Anglia and established his base of operations in the Isle of Ely, then surrounded by protective fenland. Stephen responded quickly, taking an army into the fens and using boats lashed together to form a causeway that allowed him to make a surprise attack on the isle. Nigel escaped to Gloucester, but his men and castle were captured, and order was temporarily restored in the east. Robert of Gloucester's men retook some of the territory that Stephen had taken in his 1139 campaign. In an effort to negotiate a truce, Henry of Blois held a peace conference at Bath, to which Stephen sent his wife. The conference collapsed over the insistence by Henry and the clergy that they should set the terms of any peace deal, which Stephen found unacceptable. Ranulf of Chester remained upset over Stephen's gift of the north of England to Prince Henry. Ranulf devised a plan for dealing with the problem by ambushing Henry whilst the prince was travelling back from Stephen's court to Scotland after Christmas. Stephen responded to rumours of this plan by escorting Henry himself north, but this gesture proved the final straw for Ranulf. Ranulf had previously claimed that he had the rights to Lincoln Castle, held by Stephen, and under the guise of a social visit, Ranulf seized the fortification in a surprise attack. Stephen marched north to Lincoln and agreed to a truce with Ranulf, probably to keep him from joining the Empress's faction, under which Ranulf would be allowed to keep the castle. Stephen returned to London but received news that Ranulf, his brother and their family were relaxing in Lincoln Castle with a minimal guard force, a ripe target for a surprise attack of his own. Abandoning the deal he had just made, Stephen gathered his army again and sped north, but not quite fast enough—Ranulf escaped Lincoln and declared his support for the Empress. Stephen was forced to place the castle under siege. Second phase of the war (1141–1142) While Stephen and his army besieged Lincoln