A castle (from Latin castellum) is a defensive structure symbolic of the Middle Ages. The term has a history of scholarly debate surrounding its precise meaning, but a castle is usually considered to be the "private fortified residence" of a lord or noble. This is distinct from a fortress, which was not a home, or a fortified town, which was a public defence. Over the extended period of time that castles were built they took on a great many forms with many different features, some of which, such as the curtain wall and arrowslits, were commonplace.

Castles originated in the 9th and 10th centuries, when the fall of the Carolingian Empire in Europe led to the division of the empire's territory among individual lords and princes and an emphasis on personal defence. Castles controlled their immediate areas, and were both offensive and defensive structures; they provided a base from which raids could be launched as well as protection from an enemy. Although their military origins are often emphasised, castles also served as centres of administration and symbols of power. Urban castles were used to control the local populace and important travel routes, and rural castles were often situated near elements such as a mill, integral to life in the community.

Many castles were originally built from earth and timber, but had their defences replaced later by stone equivalents. In the late 12th and early 13th century, a scientific approach to castle defence emerged. Towers proliferated, with an emphasis on flanking fire, and many new castles were polygonal, while previously they had exploited natural defences and were curvilinear, or relied on concentric defence – several stages of defence within each other that could all function at the same time, thereby maximising the castle's firepower. The origin of these changes in defence has been attributed to a mixture of influence from the Crusades – where castle technology was advanced such as the new type of concentric fortification – and drawing on earlier defences such as Roman forts for inspiration. Not all the elements of castle architecture were military in nature, and devices such as moats evolved from their original purpose of defence into symbols of power. Some grand castles had long winding approaches intended to impress, and to dominate their landscape.

Gunpowder, introduced to Europe in the 14th century, did not have an immediate impact on castle building. Castles do not show adaptation to resist bombardment by cannons until the 15th century, when artillery became powerful enough to break down walls. Although castles were built across Europe well into the 16th century, new techniques to deal with improved cannon eventually led to them becoming uncomfortable and undesirable places to live, and so true castles went into decline, replaced by artillery forts with no role in civil administration, and country houses that were indefensible. From the 18th century onwards, there was a renewed interest in castles with the construction of mock castles, part of a romantic revival of Gothic architecture, but they had no military purpose.

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