Bishop Henry took a force to Winchester where he laid siege to the royal castle which was garrisoned by Angevins. Winchester's royal castle was located on the southwest side of the city, while an episcopal castle was on the southeast side. Only two other English cities at the time had more than one castle, London with three and York with two. When she heard of the bishop's incursion Empress Matilda determined to strike back. She gathered an army of her adherents and sortied from her base at Oxford around 28 July 1141. When the empress appeared before Winchester on 31 July it was a complete surprise. Bishop Henry fled the city while his soldiers retreated to Wolvesey Castle, the one belonging to the church.