Situated along the River Medway, Rochester Castle was one of the first English castles to be rebuilt in stone, and this early stone work was carried out by Gundolf, Bishop of Rochester during the late 11th century. Renowned as a very able builder of stone structures, Gundolf was responsible for the old Rochester Cathedral, as well as the Tower of London.

In 1127, custody of ROchester Castle was given to the Archbishop of Canterbury, and remained under the same authority for the next 90 years. During this time, the huge, square keep was built from Kentish ragstone. Still standing in substantially good condition, this great tower, some 113ft (34.2m) high, consists of a basement and three floors, the second floor rising through two storeys.

The basement, a dark room used for storing goods, is lit only through small ventilation holes. On the first floor, although with the emphasis very much on defence, the evidence of fireplaces and garderobes indicate that this was an important area for conducting the business of Rochester Castle. A much grander and more open aspect is apparent on the second floor of the keep, where the Great Hall was situated, with the Great Chamber beyond - formerly believed to have been the state apartment of the archbishop. Some of the richly carved detail still exists around the fireplaces, doors and windows to give an idea of how elaborately decorated this floor would have been. Surrounding the upper part of the Great Hall is a mural gallery, with large window openings. Up again to the third floor, which consisted of another fine room giving wonderful views across the river.

Access to the keep was, traditionally, through a first floor forebuilding, an additional defence measure to protect the main part of the keep. On the second floor, above the castle entrance was the chapel, and some measure of its original splendour is still apparent in the surviving vaulting and the beautifully decorated windows.

In the early 13th century, Rochester Castle was besieged by King John, causing such damage to the south corner of the keep that it subsequently collapsed. Shortly afterwards, Rochester Castle was given the status of a major royal stronghold, and the shattered corner of the keep was reconstructed in a cylindrical style, and further protected by the addition of a drum tower. Further destruction was suffered in 1264, but repairs were not carried out for more than 100 years. During this time of neglect, coupled with being subjected to the elements, Rochester Castle began to deteriorate into ruins. However, Edward III undertook a major rebuilding and restoration programme and by 1400 Rochester Castle was, once again, a viable fortress.

By the 17th century, Rochester Castle had become neglected, the keep had been burned out, and the site was being used as a local quarry for building materials. In 1870 Rochester Castle grounds were leased to the City of Rochester, who turned them into a public park and eventually, in the 20th century, responsibility for this imposing old structure was taken over by English Heritage. Today, Rochester Castle stands as a proud reminder of the history surrounding the old city of Rochester, along with Rochester Cathedral, the cobbled streets, and the Dickensian reflections.