According to historians, the earliest recorded “castles or fortresses” were built in the late 9th century.  They included wood, earth and stone fortified structures.  During the early middle ages, Roman forts were often converted into castles.  Records indicate that as early as 537, the famous “Hadrian's Mausoleum in Rome”, was used as a fortress during the Gothic War.  Several early medieval fortresses still stand in Brescia and Trento in Italy.

During the Carolingian era, new castles were constructed in Europe for the purposes of defending state properties on the frontier.  The right to fortify was a royal privilege. In 864, Charles the Bald ordered all private fortifications erected without his permission be destroyed.

However, it is believed by historians that castle construction increased during the late 9th century.  Two contributing factors included raids by the Vikings and Magyars and a loss of centralized authority due to the downfall of the Carolingian empire.  Consequently, cities, monasteries, ports and rural settlements were fortified during this period.

At the decline of the Carolingian Empire, factions struggled for power.  These factions organized their own military infrastructures to protect their rights, their domains, and their followers. Feudalism in society began to emerge. The early castle was an integral part of Feudalism.  Not only did a Lord rely on his castle as a residence, it also provided protection for his followers.

Defensive strategies became regular practice during the times of feudalism. During the 10th century for example, in the Loire Valley, Fulk Nerra started an aggressive castle-building program to gain better control of his county of Anjou, and neighbouring Touraine.  At around the same time, a military state formed in Normandy with a concentrated network of castles and feudal allegiances.

Norman-French followers of King Edward the Confessor introduced castles to the British Isles around early 11th century.  When William the Conqueror (Duke of Normandy) executed the Norman Conquest of England, he brought with him the practice of building castles to protect and rule the land.  By this time, castle fortification was a familiar concept on the mainland of Western Europe.