At the time of the Norman Conquest there were four rapes: Arundel, Lewes, Pevensey and Hastings. Arundel and Bramber replaced Burpham and Steyning as Rapal centres. The rape of Arundel consisted of the entire area of Sussex west of the River Adur, corresponding to the boundaries of both the western division of the church in Sussex (the forerunner to the archdeaconry of Chichester) and the boundaries of the traditional western area of the Sussex dialect. By the time of the Domesday Book, William the Conqueror had created the rape of Bramber as an afterthought out of parts of the Arundel and Lewes rapes, so that the Adur estuary could be better defended. Although the origin and original purpose of the Rapes is not known, their function after 1066 is clear. With its own lord and sheriff, each Rape was an administrative and fiscal unit. The organisation of the whole of Sussex apart from royal and church lands into territorial blocks each with a fortress near the sea was exceptional. Situated between Normandy and London, control over Sussex was strategically important to William the Conqueror, who needed to protect his major communication routes. Also as the ancestral home of the last Saxon king of England, Harold Godwinson, William had to be careful to secure Sussex against revolt. William did this by dividing Sussex into territories. Under the Normans each traditional rape was now centred on a castle: Sir Henry Ellis's observation that the rapes "were military districts for the supply of the castles which existed in each" applied to the Anglo-Norman period The castles formed a network of strongholds which, as well as deterring insurgency and preventing invasion also acted as regional administrative centres. Each rape had a single sheriff and ran as a strip, north–south, from the border with Surrey/Kent to the English Channel. The castles of Arundel, Bramber and Lewes were sited on positions overlooking the rivers Arun, Adur and Ouse respectively, while those at Chichester, Hastings and Pevensey overlooked the coast.