Perhaps the first issue to be dealt with is an answer to the question, "what is a castle?" The English Medieval castle, like its counterparts in Europe, is a unique phenomenon. Most buildings are created to fulfill a single, specific purpose: a church, a house, a factory, a school, a bank, a hotel etc. A castle, depending upon the status of the man who occupied it, could be variously, a military base, a seat of government, a court and a stronghold for the surrounding region. It could be any or all of the above but it was principally the private residence of its owner, his family and his dependents. .
England had known fortifications before the advent of the castle. The Iron Age peoples of Ancient Britain fortified hilltops with massive earthworks, such as Maiden Castle in Dorset, for tribal defense. The Romans dotted the countryside with innumerable military encampments and built the impressive chain of fortresses, known as the Saxon Shore forts (e.g. Portchester Castle, below), to guard South-East England from Saxon raiders in the 2nd and 3rd centuries AD. The Normans later built castles within the walls of two of these Roman Saxon Shore forts, at Pevensey in Sussex and Portchester in Hampshire. The Anglo-Saxons and the Danes entrenched their towns behind earthen banks and timber palisades to create fortified towns, burghs, from which is derived the modern word "borough." Yet, all these structures were for, essentially, communal purposes. What distinguishes the castle from these and other, later fortifications is its function as a private residence. .
Castles were the product of that period of Medieval history termed the Age of Feudalism. Feudalism is a much misused word. It is strictly applied to the military society which was created in Europe during the 9th and 10th centuries AD and which reached its most developed form in Normandy in the 11th century. Very simply put, feudal society resembled a pyramid.