John's campaign started well. In November John retook Rochester Castle from rebel baron William d'Aubigny in a sophisticated assault. One chronicler had not seen "a siege so hard pressed or so strongly resisted", whilst historian Reginald Brown describes it as "one of the greatest [siege] operations in England up to that time". Having regained the south-east John split his forces, sending William Longespée to retake the north side of London and East Anglia, whilst John himself headed north via Nottingham to attack the estates of the northern barons. Both operations were successful and the majority of the remaining rebels were pinned down in London. In January 1216 John marched against Alexander II of Scotland, who had allied himself with the rebel cause. John took back Alexander's possessions in northern England in a rapid campaign and pushed up towards Edinburgh over a ten-day period.