Encastellation (sometimes castellation, which can also mean crenellation) is the process whereby the feudal kingdoms of Europe became dotted with castles, from which local lords could dominate the countryside of their fiefs and their neighbours', and from which kings could command even the far-off corners of their realms.
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 Colosseum.
In the Mezzogiorno, the independent principalities of the Lombards and the Italian city-states, which distanced themselves from any central authority, formed an opportune place for the proliferation of castles.
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.
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.
[4] 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 castles first began to spread quickly in the tenth century, in light of the increasing power of the Castilian counts vis-à-vis the king.
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.
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.
The first castles were motte-and-baileys built on the expanding frontier of the English Pale and within it to control the local population, according to Gerald of Wales.
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.