 Situated on the River Medway and Watling Street , Rochester served as a strategically important royal castle . During the late medieval period it helped protect England 's south @-@ east coast from invasion . The first castle at Rochester was founded in the aftermath of the Norman Conquest . It was given to Bishop Odo , probably by his half @-@ brother William the Conqueror . During the Rebellion of 1088 over the succession to the English throne , Odo supported Robert Curthose , the Conqueror 's eldest son , against William Rufus . It was during this conflict that the castle first saw military action ; the city and castle were besieged after Odo made Rochester a headquarters for the rebellion . After the garrison capitulated , this first castle was abandoned . 
