A medieval European structure, generally the residence of a king or a lord of the territory. The word 'castle' is derived from Latin 'castellum', a fortified camp, and there are various linguistic forms, including chateau, castello, castrum, and burg. These medieval strongholds developed rapidly from the 9th century. The word is sometimes applied to prehistoric earthworks, such as Maiden Castle, England. Castles developed with the feudal system which installed a societal classification in which land and other privileges were granted in return for military service. Castle architecture had three essential elements: a tower (keep or donjon), residence for the noble, and a fortified enclosure wall. The first late Carolingian types were likely modeled on the fortified homesteads of the Slavs, and in the 10th century the manor or principal house was then set up on a raised mound within the enclosure. This motte and bailey type was introduced to France in the 11th century. The Normans then it to the British Isles and southern Italy and also built stone keeps within their enclosures. Later 12th-century castles in France and England have large stone walls gateways modeled on Arabic and Byzantine forts and massive circular central keeps. Multiple walls with strengthened gateways are an invention of the mid-13th century. The introduction of the cannon and other firearms in the 15th and 16th centuries made castles vulnerable to attack. Castle architecture was revised with low walls which could be defended all around by artillery the guns mounted on bastions and redans.