 In 1127 King Henry I granted Rochester Castle to the Archbishop of Canterbury , William de Corbeil , and his successors in perpetuity . He was given permission to build " a fortification or tower within the castle and keep and hold it forever " . Corbeil is responsible for building the great tower or keep that still stands today , albeit in an altered state . The 12th century saw many castles in England rebuilt in stone , an advancement in sophistication of design and technology . Although Rochester had already been given a stone curtain wall by Bishop Gundulf , the Keep dates from this period . It visually dominated the rest of the castle , towering above its outer walls , and acted as a residence containing the castle 's best accommodation . A sturdy fortification , it could also serve as a stronghold in the event of military action . Such was the importance of the keep as a symbol of Rochester it was depicted on the town 's seal in the 13th century . 
