Castellum and castrum are terms from Roman military architecture. A castrum was a large fortified legionary camp, while a castellum was a smaller auxiliary fort. Both followed a standardised layout that became the foundation for countless European cities.
Design and Distinction
A castrum served as a permanent base for an entire Roman legion, typically housing 5,000 to 6,000 soldiers. It featured a rectangular plan enclosed by an earthen rampart (vallum), a ditch, and a timber or stone wall. Inside, two main roads intersected: the via principalis and the via praetoria. At the crossroads stood the headquarters building (principia), which housed the commanding officer, administrative offices, and the legion’s standards.
A castellum was smaller in scale, accommodating a cohort or ala of 500 to 1,000 troops. It followed the same planning principles as the castrum but in a reduced format. Castella were commonly built along the empire’s frontiers (limes) as part of an integrated defence system.
Key architectural features of both types include:
- Rectangular plan with rounded corners
- Four gates, one on each side
- Internal layout with barracks, granaries (horrea), bathhouses, and a hospital
- Defensive towers at regular intervals along the walls
Application
The influence of castra and castella on urban development has been profound. Numerous European cities grew directly from Roman military camps. Cities such as Cologne (Colonia), Vienna (Vindobona), Regensburg (Castra Regina), and Utrecht (Trajectum) all trace their origins to Roman fortifications. The grid-pattern street layouts still visible in many historic city centres are a direct legacy of Roman camp planning.
In modern architecture and archaeology, the study of castra and castella informs heritage conservation and urban renewal projects in cities built on Roman foundations.
Related terms
- Limes
- Fortification
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.