an initial attempt to capture a port to receive the Empress Matilda's invading army, but Stephen's forces forced him to retreat into the south-west. The following month, however, Henry I's widow, Adeliza, invited her stepdaughter to land at Arundel instead, and on 30 September Robert of Gloucester and the Empress arrived in England with 140 knights. The Empress stayed at Arundel Castle, whilst Robert marched north-west to Wallingford and Bristol, hoping to raise support for the rebellion and to link up with Miles of Gloucester, a capable military leader who took the opportunity to renounce his fealty to the King. Stephen promptly moved south, besieging Arundel and trapping Matilda inside the castle. Stephen then agreed to a truce proposed by his brother Henry; the full details of the truce are not known, but the results were that Stephen first released Matilda from the siege and then allowed her and her household of knights to be escorted to the south-west, where they were reunited with Robert. The reasoning behind Stephen's decision to release his rival remains unclear. Contemporary chroniclers suggested that Henry argued that it would be in Stephen's own best interests to release the Empress and concentrate instead on attacking Robert, and Stephen may have seen Robert, not the Empress, as his main opponent at this point in the conflict. He also faced a military dilemma at Arundel—the castle was considered almost impregnable, and he may have been worried that he was tying down his army in the south whilst Robert roamed freely in the west. Another theory is that Stephen released Matilda out of a sense of chivalry; he was certainly known for having a generous, courteous personality and women were not normally expected to be targeted in Anglo-Norman warfare. Having released the Empress, Stephen focused on pacifying the south-west of England. Although there had been few new defections to the Empress, his enemies now controlled a compact block of territory stretching out from Gloucester and Bristol south-west into Devon and Cornwall, west into the Welsh Marches and east as far as Oxford and Wallingford, threatening London. Stephen started by attacking Wallingford Castle, held by the Empress's childhood friend Brien FitzCount, only to find it too well defended. He then left behind some forces to blockade the castle and continued west into Wiltshire to attack Trowbridge Castle, taking the castles of South Cerney and Malmesbury en route. Meanwhile, Miles