The main function of all castles was defense, everything else was secondary. They were always surrounded by a curtain wall, which was often supplemented by a reinforced shield wall at strategic points. Crenellated battlements and arrow slits protected the defenders, and attackers often also had to overcome a series of several gates. The main entrance was protected by a drawbridge and machicolations projecting over the gate, from which boiling liquids and missiles could be dropped on the hapless foes below. The final refuge of the castle residents was the stronghold, or keep. It was the tallest and strongest building within the walls, with a high entrance accessible only via a removable ladder or wooden bridge. In addition to being a watchtower and the centre point of the entire castle, the keep was also a status symbol. The main residential building (great hall) was called the Palas. The castle chapel was often installed in the gatehouse or one of the main towers (nearer my God to Thee!), and a small garden inside the walls provided herbs, flowers and vegetables in emergencies. Offices and service rooms were generally located in the outer ward.

From the 16th Century on, castle walls had to be made increasingly thicker and stronger in response to the development of artillery. Only a few castles were made into mighty fortresses. Instead, the nobility increasingly moved to more comfortable residential castles, Many, were sumptuously ostentatious and magnificent buildings

During the third century B.C., the art of war made rapid progress so the defense tactics were improved. Romans and Greeks already used gateways in the defense of a fortress; curtain walls and towers were also common. The Romans governed for so long and occupied a vast empire. They left rectangular buildings measuring about 60ft by 70ft called milecastle. 

The medieval castle originated in the ninth century in France, western Germany and northern Italy because the nobles began building fortifications in response to increasing insecurity in the region. 

In Europe in the Middles Ages the castle was a fortified residence of a nobleman, effectively displaying his authority and feudal lordship over the territory associated with it as well as serving a military function. The castles were sometimes fortified towns with a fortified bridge which was very important against the defense of the Vikings.

Residential
The castle was a status symbol, the element of display was  never absent so from the beginnings the trappings of fortification such as ditches, banks and mounds, towers, gatehouse and crenellations were sometimes more for show than for a serious purpose. 

ARCHITECTURE

Motte-and-bailey castles.
Not all early castles were of earthwork and timber only; some belong to the category of “motte-and-bailey”. The main enclosure was the bailey, defended by a ditch, bank and palisade, with a timber gate or gate tower; the bailey contained all the residential buildings, presumably timber-framed, required by a lordly household, at least in the first generation. To this was added the motte, usually to one side with direct access to the open country, the whole looking like a figure of eight, with the area of the bailey larger than that of the motte. The motte itself was a great mound of earth, usually artificial, though sometime part-natural, with its own ditch and bank about the base and thus separated from the bailey, from which it summit was reached usually by an inclined and stepped timber bridge. In the classic model a timber palisade with a gateway crowned the flattened summit of the mound and within this there rose a timber tower. 

Ring works
These are earthen mounds, which supported a timber superstructure, sometimes combined with a bailey or outer-banked enclosure. It is the method of fortification applied to towns and cities and was the earliest type of castle.  During the 12th century the great tower of stone succeeded the motte as the most favored type of donjon, it is sometimes equally notable by its absence. In a feudal age so dependent upon hierarchy, status and symbolism, it may be difficult to conceive of a castle with no donjon of any kind. Sometimes these castles would have an inner bailey containing the private apartments as an inner sanctum of lordship or regality, originally emphasized and embellished architecturally. 

Tower Keeps
It is a self-contained strong house, a defensible seigniorial   residence usually with more than one suite of accommodation, a residence that would also be the whole castle. It had all the components, halls, chambers, chapels, and sometime kitchens all together into one compact unit, the result being an impressive tower. There were rectangular towers divided from top to bottom by a cross wall that gave structural strength to the whole and facilitated roofing. The principle suite do the lord himself was marked by extra ornamentation and sometimes rose through two stories to give extra space and light. The majority of those built in the 12th. Century were rectangular, but later the cylindrical donjon became fashionable, especially in France. This change from rectangular to cylindrical was a symbol of progress, the round tower being stronger in order to withstand the battering of missiles or the assault of pick and bore, and avoiding blind angles for the defenders.

Shell Keeps
The shell keep in its classic form is a ring, sometimes polygonal,replacing the presumed original timber palisade at the summit of the motte. This produced an enclosed circular courtyard, in which stone or wooden buildings were constructed, often butted up against the curtain wall. Sometimes the circular wall was built not on top of the motte but around its base.

Mural Towers.
The mural or flanking tower which by its projection and superior height made possible the adequate defense of long lengths of curtain wall from its arrow slits and fighting top. Early towers are always rectangular, later towers 13th. Century onwards is cylindrical or D-shaped.

Gateways
Castle gates also depend for their defense upon the exploitation of the mural tower as potential weak points likely to be attacked first.  The towers could be sealed off so that if attackers gained possession of one section, they could not necessarily continue to the next without capturing the tower.  The gateway into the bailey was generally a substantial building of two or three stories and was approached from across the moat by means of a causeway and a drawbridge. In addition to the main gateway there was usually a small postern, placed in such a position that escape could not be affected or a sally made unobserved by the enemy.

Concentric Castles 
Is often regarded as the ultimate in perimeter defense. It consists of one towered curtain within another, the outer commanded by the loftier and stronger inner curtain and serving, like the moat, to keep the enemy at bay.