The process was rather quick once the castle, as a distinct type of fortress, was introduced. However, it took different forms in different lands. The methods and reasons of encastellation differed based on law (who could legally build a castle), necessity (who needed a castle), and geography (where could castles be effectively built). The stone castle originated probably in the north of France in the tenth century. Older wooden castles, of the motte-and-bailey variety are probably older, though they were far more common until well into the twelfth century.
Because [Hugh of Abbeville's peers] were not all lords of castles, [he] became more powerful than the rest of his peers. For he could do what he liked without fear, relying on the protection of the castle, while others, if they tried anything, were easily overcome as they had no refuge.
From Normandy and Anjou, encastellation spread to the Loire Valley. In Poitou, there were thirty nine castles by the eleventh century, the constructions primarily of local magnates. Fortification had briskly increased in Gaul during the Viking Age (see Edict of Pistres) and this merely continued apace while the Carolingian Dynasty declined in importance and regional control devolved to regional lords.
In Languedoc and the south of France, there were more serious attempts, the Peace and Truce of God movements, to curb feudal warfare. But with the spread of heresy came the spread of castles as fortresses to which heretic barons could flee, such as the "five sons of Carcassonne."
In the north, the castles were originally the seats of the barons. They spread quickly after the disruption of royal authority in Italy in the mid-tenth century. By the eleventh century, the territorial magnates, like the margrave of Tuscany, were supreme and castles dotted the landscape. With the rise of the city-states after the collapse of Tuscan power in the early twelfth century, the powerful merchant families began to construct fortress and towers as residences in the cities. Well-preserved San Gimignano was the result of the struggle between Guelph and Ghibelline.
In the centre of the peninsula, the Papal States, the agents of encastellation were not large territorial magnates, but the petty nobles who belonged to various families and factions usually associated with Rome in some way. The Crescentii and the Tusculani constructed fortresses throughout Latium to dominate the roads leading to the Eternal City and the Vatican. During the papal nadir of the tenth and eleventh centuries, their hilltop fortresses gave these minor lords far more power than their territories would otherwise permit. In Rome itself, encastellation often led to the fortifying of the ancient monuments which had fallen into disuses, such as the Arch of Constantine and the Coliseum. These fortresses were usually in the hands of one of the powerful lay families, but sometimes of the popes.
In the Mezzogiorno, the independent principalities of the Lombards and the Greek city-states, which distanced themselves from any central authority, formed an opportune place for the proliferation of castles. Indeed, the nominally Byzantine duchies of Gaeta, Naples, and Amalfi grew around what were originally small coastal fortresses. The decline of ducal authority in these places has been blamed on the tendency to give outlying regions to younger sons (e.g. Docibilis II of Gaeta granting Fondi to Marinus), who then built their own fortresses and thus became independent in fact. Historian G. A. Loud considers incastellamento as one of the chief reasons for the decline in princely influence in Benevento and Capua (especially the former) during the late tenth century. Historian Barbara Kreutz notes the encastellation of the monastic estates which dominated south Italian politics and contributed to the constant confiscation and invasion of monastic estates as lay barons sought to increase their power against their foes during the war-filled eleventh and twelfth centuries. The arrival of the Normans, adept castle-builders, in the early eleventh century only exacerbated the tendency toward fortification of every hilltop. Together with the Prince of Salerno, they subdued Calabria and encastellated its mountainous territory, leading to the inevitable invasion of Sicily.
Encastellation began in earnest under William the Conqueror in the years immediately following the Battle of Hastings. Castles of the motte-and-bailey were erected quickly all over the country to subdue the locals and prevent foreign invasions by rival claimants to the throne. Within a matter of years, England was fully castled. Most of these castles belonged to the king or one of his tenants-in-chief. The construction of numerous castles by minor lords was a feature, as in most place, of the reign of weaker kings. After the iron hand of William's sons had passed, Stephen took the throne and the Anarchy (1135 – 1152) of civil war which characterised his reign saw the proliferation of adulterine (unauthorised) castles: to the number of 1,115, according to one chronicler.
Motte-and-bailey castles existed from before the thirteenth century in those parts of Wales which fell under English authority and they spread in south Wales after its conquest, but the most famous encastellation of Wales occurred in the north under Edward I of England (1272 – 1307). His famous Edwardian concentric castles, large stoneworks with multiple rings of defences, grew up at strategic locations throughout the north and the local populace was placed securely under English authority. In this case, encastellation was the result, not of weak central authority, but of a strong royal hand and direction.
In Prussia, during the Drang nach Osten and the Northern Crusades of the twelfth and thirteenth centuries, encastellation was the result of the Margraves of Brandenburg and the Teutonic Knights, who, among others, conquered the land from the pagan Prussians. The construction of castles to control territories occurred at a late point in the development of the castle and these fortresses were large and complex. They were called Ordensburgen and they served as headquarters and training grounds for initiates into the knightly orders.