control of their adversaries' territory and ultimately win a slow, strategic victory. The armies of the period centred on bodies of mounted, armoured knights, supported by infantry and crossbowmen. These forces were either feudal levies, drawn up by local nobles for a limited period of service during a campaign, or, increasingly, mercenaries, who were expensive but more flexible and often more skilled. These armies, however, were ill-suited to besieging castles, whether the older motte-and-bailey designs or the newer, stone-built keeps. Existing siege engines were significantly less powerful than the later trebuchet designs, giving defenders a substantial advantage over attackers. As a result, slow sieges to starve defenders out, or mining operations to undermine walls, tended to be preferred by commanders over direct assaults. Occasionally pitched battles were fought between armies but these were considered highly risky endeavours and were usually avoided by prudent commanders. The cost of warfare had risen considerably in the first part of the 12th century, and adequate supplies of ready cash were increasingly proving important in the success of campaigns. Stephen's personal qualities as a military leader focused on his skill in personal combat, his capabilities in siege warfare and a remarkable ability to move military forces quickly over relatively long distances. In response to the revolts and invasions, he rapidly undertook several military campaigns, focusing primarily on England rather than Normandy. His wife Matilda was sent to Kent with ships and resources from Boulogne, with the task of retaking the key port of Dover, under Robert's control. A small number of Stephen's household knights were sent north to help the fight against the Scots, where David's forces were defeated later that year at the battle of the Standard in August by the forces of Thurstan, the Archbishop of York. Despite this victory, however, David still occupied most of the north. Stephen himself went west in an attempt to regain control of Gloucestershire, first striking north into the Welsh Marches, taking Hereford and Shrewsbury, before heading south to Bath. The town of Bristol itself proved too strong for him, and Stephen contented himself with raiding and pillaging the surrounding area. The rebels appear to have expected Robert to intervene with support that year, but he remained in Normandy throughout, trying to persuade the Empress Matilda to invade England herself. Dover finally surrendered to the queen's forces later in the year. Stephen's military campaign in England had