A DETAILED STUDY OF THE ENGLISH CASTLES OF THE MEDIEVAL PERIOD. The simple castles raised after the Norman conquest had been developed throughout 11th and 12th centuries, whilst the introduction of Islamic and Byzantine fortification techniques from the late 12th century led to further developments in castle architecture. These fortifications were to be well tested throughout the course of the 13th century as England was riven by conflict, characterized by prolonged sieges, between the monarchy and powerful magnates. As this book reveals, as well as providing the focus for warfare, castles increasingly became the centres of their communities. They provided a more permanent base for the lord, his family and retainers, as well as acting as centres for justice and administration.