Rochester Castle in the context of William Rufus

Rochester (/ˈrɒtʃɪstər/ ROTCH-iss-tər) is a port town in the unitary authority of Medway, in Kent, England. It is at the lowest bridging point of the River Medway, about 30 miles (50 km) east-southeast of London. The town forms a conurbation with neighbouring towns Chatham, Rainham, Strood and Gillingham. Rochester was a city until losing its status as one in 1998 following the forming of Medway and failing to protect its status as a city, the first city to do so in the history of the United Kingdom. There have been ongoing campaigns to reinstate the city status for Rochester. In 2011 it had a population of 62,982.

Rochester was for many years a favourite of Charles Dickens, who owned nearby Gads Hill Place, Higham, basing many of his novels on the area. The Diocese of Rochester, the second-oldest in England, is centred on Rochester Cathedral and was responsible for founding a school, now The King's School, in 604 AD, which is recognised as the second-oldest continuously running school in the world. Rochester Castle, built by Bishop Gundulf of Rochester, has one of the best-preserved keeps in either England or France. During the First Barons' War (1215–1217) in King John's reign, baronial forces captured the castle from Archbishop Stephen Langton and held it against the king, who then besieged it.

A keep is a type of fortified tower built within castles during the Middle Ages by European nobility. Scholars have debated the scope of the word keep, but usually consider it to refer to large towers in castles that were fortified residences, used as a refuge of last resort should the rest of the castle fall to an adversary. The first keeps were made of timber and formed a key part of the motte-and-bailey castles that emerged in Normandy and Anjou during the 10th century; the design spread to England, Portugal, south Italy and Sicily. As a result of the Norman Conquest of England in 1066, use spread into Wales during the second half of the 11th century and into Ireland in the 1170s. The Anglo-Normans and French rulers began to build stone keeps during the 10th and 11th centuries, including Norman keeps, with a square or rectangular design, and circular shell keeps. Stone keeps carried considerable political as well as military importance and could take a decade or more to build.

During the 12th century, new designs began to be introduced – in France, quatrefoil-shaped keeps were introduced, while in England polygonal towers were built. By the end of the century, French and English keep designs began to diverge: Philip II of France built a sequence of circular keeps as part of his bid to stamp his royal authority on his new territories, while in England castles were built without keeps. In Spain, keeps were increasingly incorporated into both Christian and Islamic castles, although in Germany tall fighting towers called bergfriede were preferred to keeps in the western fashion. In the second half of the 14th century, there was a resurgence in the building of keeps. In France, the keep at Vincennes near Paris began a fashion for tall, heavily machicolated designs, a trend adopted in Spain most prominently through the Valladolid school of Spanish castle design. Meanwhile, tower keeps in England became popular amongst the most wealthy nobles: these large keeps, each uniquely designed, formed part of the grandest castles built during the period.

Gundulf (or Gundulph) (c. 1024 – 1108) was a Norman monk who went to England following the Norman Conquest of 1066. He was appointed Bishop of Rochester and Prior of the Cathedral Priory there. He built several castles, including Rochester, Colchester and the White Tower of the Tower of London, and the Priory and Cathedral Church of Rochester.