Many people believe that castles were built into modern times, however, that is a fallacy. In Britain, true castles were built only during the Middle Ages and the beginning of the Renaissance. On the Continent, they continued to be built a bit longer, but not much.
What is a real castle? Simply put, a castle is a "properly fortified military residence." As such, it housed the lord and his family and - at the same time - functioned in a military role, dominating an area, subjugating a local population, and garrisoning a group of soldiers. These men often functioned as mini-kings; but, ultimately, they owed allegiance to the monarch and were required to compensate the king for the right to maintain a castle and take advantage of the local economy.
The construction of castles began on mainland Europe well before they made their appearance in Britain in the 11th century. When Duke William of Normandy decided to assert his right to the English throne in 1066, one of the first tasks he set for his army was the construction of a castle inside the Roman fortifications at Pevensey, on England's southern coast. There, he landed with his fleet to begin his assault against Harold Godwinson, the Anglo-Saxon nobleman who became king upon Edward the Confessor's death. As the Bayeux Tapestry depicts, William was also responsible for the construction of several other castles, including the one at Hastings, near the battlefield where he defeated Harold II to become King William I.
In order to ensure his control over his new kingdom, William swiftly introduced feudalism to England and ordered his feudal lords to construct scores of castles to subjugate the populace and symbolically establish the dominance of the Normans. The earth and timber strongholds, known as motte and bailey castles and ringworks, were inexpensive and fairly easy to build with locally available materials and unskilled laborers. They could be constructed in a matter of weeks, if needed, and made the ideal weapon to solidify Norman rule. However, their tendency to rot and burn easily was a key reason why stone castles soon became the dominant building type.
Castles meant different things to different builders. The extent of fortification, the layout of the internal structures, the choice of location, and the grandiosity of ornamentation all reflected the needs, expectations and idiosyncrasies of the men who built and lived in them. Castles were much more than defensive structures built to protect the lord and residents. Castles were much more than grand homes for the monarch or regional ruler. Indeed, they were much more than merely functional facilities; otherwise, they could have easily been built to a standard plan. While the structure itself was built to contain all the buildings the lord considered as essential to the operation of his lordship, how the buildings were laid out - the image they presented to others - was as carefully, sometimes obsessively, contemplated as their ability to provide life's daily military and domestic requirements.
Britain's medieval castles served individual overlords, who resided inside perhaps only part of the year but who retained control over the surrounding countryside, reaping the economic benefits of possessing the lordship and manorial estates. Only occasionally in times of war were these fortified private residences used as refuges for the local population. Not only did the construction of castles reiterate that the Normans now controlled the kingdom but their permanent presence in the landscape also emphasized the increasing responsibility placed on the individual leader, the commander of a region, whose separate status from the general populace and prestigious political position as the monarch's representative warranted a dwelling worthy of that status, one that provided distinct living arrangements as well as defensive might - the castle.