One of the secrets behind the success of the Norman conquest was the use of a system of control and defence, imported from Normandy, based on a defensible stronghold known as a castle. The castle provided an imposing and effective defensive base where military forces could be housed, as well as a stronghold which could be held against attack or siege, but it was used to support an aggressive policy of control rather than a passive defensive system.
The earliest castles of the Norman period were often motte-and-bailey castles, of which there are examples across Essex, although one of the best preserved is at Pleshey. The keep, a tower of wood or stone, sat on top of a large mound or motte. A lower defended area, the bailey, housed accomodation and space for more domestic functions. Both motte and bailey were surrounded by one or more defensive ditches.
The form of castles changed through the medieval period, partly due to advances in military technology which required a change in the defences. The defensive focus shifted from the great keep of the Norman period, as evidenced at Castle Hedingham, to the system of a curtain wall with towers which can be seen for example at Hadleigh. By the end of the medieval period the emphasis of castle building was no longer on defence but on building an impressive and luxurious residence, and this demonstrates not only the changing nature of warfare, but also the changes in society and in increasing political stability, which made social competition as important as defensive capability.