escape to Wallingford. The garrison surrendered shortly afterwards, but Stephen had lost an opportunity to capture his principal opponent. Stalemate (1143–1146) The war between the two sides in England reached a stalemate in the mid-1140s, while Geoffrey of Anjou consolidated his hold on power in Normandy. 1143 started precariously for Stephen when he was besieged by Robert of Gloucester at Wilton Castle, an assembly point for royal forces in Herefordshire. Stephen attempted to break out and escape, resulting in the battle of Wilton. Once again, the Angevin cavalry proved too strong, and for a moment it appeared that Stephen might be captured for a second time. On this occasion, however, William Martel, Stephen's steward, made a fierce rear guard effort, allowing Stephen to escape from the battlefield. Stephen valued William's loyalty sufficiently to agree to exchange Sherborne Castle for his safe release—this was one of the few instances where Stephen was prepared to give up a castle to ransom one of his men. In late 1143, Stephen faced a new threat in the east, when Geoffrey de Mandeville, Earl of Essex, rose up in rebellion against him in East Anglia. The King had disliked the Earl for several years, and provoked the conflict by summoning Geoffrey to court, where the King arrested him. He threatened to execute Geoffrey unless the Earl handed over his various castles, including the Tower of London, Saffron Walden and Pleshey, all important fortifications because they were in, or close to, London. Geoffrey gave in, but once free he headed north-east into the Fens to the Isle of Ely, from where he began a military campaign against Cambridge, with the intention of progressing south towards London. With all of his other problems and with Hugh Bigod, 1st Earl of Norfolk, in open revolt in Norfolk, Stephen lacked the resources to track Geoffrey down in the Fens and made do with building a screen of castles between Ely and London, including Burwell Castle. For a period, the situation continued to worsen. Ranulf of Chester revolted once again in the summer of 1144, splitting up Stephen's Honour of Lancaster between himself and Prince Henry. In the west, Robert of Gloucester and his followers continued to raid the surrounding royalist territories, and Wallingford Castle remained a secure Angevin stronghold, too close to London for comfort. Meanwhile, Geoffrey of Anjou finished securing his hold on southern Normandy and in January