In 1087, Gandulf, Bishop of Rochester, started the construction of Rochester Castle at a strategic site guarding the crossing of the River Medway on the main route to the Channel ports. The massive stone keep, the largest Norman keep in the country, was built by William de Corbeil, Archbishop of Canterbury, to whom Henry I granted custody of the castle in 1127. The south west corner tower is cylindrical rather than square, built as a replacement to the original tower that collapsed during a siege in 1215. Just months after the signing of the Magna Carta the castle was held by rebellious barons. King John led a determined attack on the castle, using miners to dig under the tower until only their wooden pit props were holding it up. They then built a bonfire using the fat from forty pigs to feed the flames and burn the wooden props, causing the tower to collapse.