The Roman Castrum is a strong defensive building that can be garrisoned by troops. Garrisoned units add to the firepower of the Castrum. You may also train the Legionary, Saggittarius, Manuballista, Light Auxiliary and Scorpio from the Castrum.

In the Roman Republic and the Roman Empire, the Latin word castrum (plural castra) was a building, or plot of land, used as a fortified military camp. In English, the terms 'Roman fort', 'Roman camp' and 'Roman fortress' are commonly used for castrum. However, scholastic convention tends toward the use of the words 'fort', 'camp', 'marching camp' and 'fortress' as a translation of castrum.

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