Castra hiberna

In the Roman Empire, the Latin word castrum (plural castra) was a building, or plot of land, used as a fortified military camp.

Castrum was the term used for different sizes of camps including a large legionary fortress, smaller auxiliary forts, temporary encampments and "marching" forts. The diminutive form castellum was used for fortlets, typically occupied by a detachment of a cohort or a century.

In English, the terms "Roman fortress", "Roman fort" and "Roman camp" are commonly used for castrum. However, scholastic convention tends toward the use of the word "camp", "marching camp" and "fortress" as a translation of castrum.

For a list of known castra see List of castra.

The term castrum appears in three Italic languages: Oscan, Umbrian and Latin. In Latin castrum was probably originally the term for an estate or a tract of land enclosed by a fence or a wooden or stone wall of some kind, as seems to be used in a few passages in Cornelius Nepos' works.

Considering that the earliest military shelters were tents made of hide or cloth, and all but the most permanent bases housed the men in barracks of tents placed in quadrangles and separated by numbered streets, a castrum may originally have been a tent.

The commonest Latin syntagmata for the term castra are:

In Latin the term castrum is much more frequently used as a proper name for geographical locations: e.g. Castrum Album, Castrum Inui, Castrum Novum, Castrum Truentinum, Castrum Vergium. The plural was also used as a place name, as Castra Cornelia, and from this come the Welsh place name prefix "Caer" and English suffixes "Caster" and "Chester"; e.g. Winchester, Lancaster.

Castrorum Filius was one of names used by the emperor Caligula and then also by other emperors.

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