The rebels made the first move in the war, seizing the strategic Rochester Castle, owned by Langton but left almost unguarded by the archbishop. John was well prepared for a conflict. He had stockpiled money to pay for mercenaries and ensured the support of the powerful marcher lords with their own feudal forces, such as William Marshal and Ranulf of Chester. The rebels lacked the engineering expertise or heavy equipment necessary to assault the network of royal castles that cut off the northern rebel barons from those in the south. John's strategy was to isolate the rebel barons in London, protect his own supply lines to his key source of mercenaries in Flanders, prevent the French from landing in the south-east, and then win the war through slow attrition. John put off dealing with the badly deteriorating situation in North Wales, where Llywelyn the Great was leading a rebellion against the 1211 settlement.