The list of castra used by the Roman army in the Republic and Empire includes castra in various places of Europe, Asia and Africa. The Latin word castra, with its singular castrum, was used by the ancient Romans to mean buildings or plots of land reserved to or constructed for use as a military defensive position.
The disposition of the castra reflects the most important zones of the empire from a military point of view. Many castra were disposed along the Northern European frontier, in Central Europe, and in modern Bulgaria and Romania, Another focal point was the Eastern border, where the Roman Empire confronted with its longest enemy, the Persian Empire. Other castra were located in strategically important zones, as in Egypt, from which most of the wealth consumed in the empire came. Finally, other castra were located in zones in which the Romans experienced local unrest, such as Northern Spain and Judea. Provinces where the Roman power was unchallenged, such as Italy, Gaul, Africa and Greece, were provided with few or no castra.
In the long history of the Roman Empire, the character of the military policy of the Roman Empire changed, and consequently the location and dimension of the castra changed. Under Emperors Gallienus and Aurelian (and later Diocletian), the Roman army was organized into a high-mobility central army (the comitatus) and in local troops (the limitanei). Some castra lost importance, others were built in new zones, and in general they lost the role of permanent quarter for huge corps of troops.