Most European Castles were made out of wood until the 12th Century, even the original Moscow Kremlin, built in 1156, was built using pine and not enlarged and rebuilt out of stone until 1380.
In its simplest terms, the definition of a castle accepted amongst academics is "a private fortified residence". This contrasts with earlier fortifications, such as Anglo Saxon burhs and walled cities such as Constantinople and Antioch in the Middle East; castles were not communal defences but were built and owned by the local feudal lords, either for themselves or for their monarch. Feudalism was the link between a lord and his vassal where, in return for military service, the lord would grant the vassal land and expect loyalty. In the late 20th century, there was a trend to refine the definition of a castle by including the criterion of feudal ownership, thus tying castles to the medieval period, however, this does not necessarily reflect the terminology used in the medieval period. During the First Crusade (1096–1099) the Frankish armies encountered walled settlements and forts that they indiscriminately referred to as castles, but which would not be considered as such under the modern definition.