From as early as Neolithic times (between 8500 BC-2500 BC), people built hill forts to protect themselves. Many earthworks survive today, along with evidence of palisades to accompany the ditches. The Romans commonly encountered hill forts built by their enemies. Though primitive, they were often effective and required extensive siege engines and other siege warfare techniques to overcome, such as at the Battle of Alesia. The Romans’ own fortifications (castra) varied from simple temporary earthworks thrown up by armies on the move, to elaborate permanent stone constructions, notably the milecastles of Hadrian’s Wall. Roman forts were generally rectangular with rounded corners. The Roman engineer Vitruvius was the first to note the three main advantages of round corner towers: more efficient use of stone, improved defence against battering rams and improved field of fire. It was not until the 13th century that these advantages were rediscovered.

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A castle was not only a bastion and place for detention of prisoners but also a social place where a knight or lord could entertain his peers. Over time the aesthetics of the design increased in importance, as the appearance and size began to reflect the prestige and power of the occupant.

Castles were built as defensive measures and offensive weapons, but often over time comfortable homes evolved within the fortified walls. An example is the Windsor Castle, first built as a Norman Conquest fortress; today a home to the Queen of the United Kingdom. The Alhambra in Al-Andalus incorporated both defensive and residential features, but after the Reconquista unified Spain, its importance shifted and it became a palace under Charles V.

The chief distinguishing features of castles, as opposed to other defensive structures, can be defined as follows:

• Castles were places of protection from an invading enemy, a place of retreat. This is the purpose behind such stereotypical castle features as portcullises, battlements and drawbridges.

• Castles were also offensive structures, built in otherwise hostile territories from which to exert strategic control over surrounding lands, as forward camps from which to conduct offensives. In particular, during the High Middle Ages, castles were often built for territorial expansion and regional control. A castle was a stronghold from which its occupants could control surrounding territory.

• Castles were either built as, or evolved into, residences for the monarch or their feudal vassals who built them.

These three purposes distinguish the castle from other fortresses - which are usually purely defensive (like citadels and city walls) or purely offensive (a military camp) - or edifices that are entirely residential in nature, like palaces. Castles such as the Tower of London served as prisons.

Castle comes from the Latin word castellum. This is a diminutive of the word castrum, which means “fortified place”. The word “castle” (Castell) was introduced into English shortly before the Norman Conquest to denote this type of fortress, then new to England, brought in by the Norman knights. In Spain, a fortified dwelling on a height for the administering authority retains its Moorish name of alcázar, whilst shiro also figure prominently in Japanese history, where the feudal daimyō inhabited them.

A French castle is a Château-Fort, for in French a simple château connotes a grand country house at the heart of an estate, with non-military, purely residential function. When European castles were opened up and expanded into pleasure dwellings and power houses from the late 15th century, their “castle” designations, relics of the feudal age, often remained attached to the dwelling, resulting in many non-military castles and châteaux.

In Germany there are two names for what would be called a castle in English, Burg (Burh) and Schloss. A Burg is a medieval structure of military significance, while a Schloss was built after the Middle Ages as a palace and not for defensive purposes.

In Celtic countries, Caer or castell (Welsh), dún and caisleán (Irish), dùn and caisteal (Scots Gaelic) are used.

In spite of the generally accepted definition, the word “castle” is sometimes used to mean a citadel (such as the castles of Badajoz and Burgos) or small detached forts d’arrêt in modern times and, traditionally, in Britain it has also been used to refer to prehistoric earthworks (e.g. Maiden Castle). The use of the Spanish equivalent castillo can be equally misleading, as it can refer to true castles and forts (eg. Castillo de San Marcos); terms such as Fortaleza (”fortress”) are in similar situations.

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A castle is a defensive structure seen as one of the main symbols of the Middle Ages. The term has a history of scholarly debate surrounding its exact meaning, but it is usually regarded as being distinct from the general terms fort or fortress in that it describes a residence of a monarch or noble and commands a specific defensive territory.

Roman forts and hill forts were the main antecedents of castles in Europe, which emerged in the 9th century in the Carolingian Empire. The advent of cannon and gunpowder changed the needs of warfare in Europe, limiting the effectiveness of the castle and leading to the rise of the fort.

Similar constructions in Russia (Kremlin) and feudal Japan (Shiro) are also considered castles. via wikipedia