Legion Headquarters – The Principia Legionis

The administrative and religious heart of the legion was its headquarters, the principia legionis.


The principia was situated at the juncture of the via Praetoria and the via Principalis in a camp or fort.


In the marching camps of the early Republic the principia and the praetorium (the commander’s quarters) were combined. In permanent installations the principia and the praetorium were separate entities.


The front of the principia ran along the via Principalis. In a legionary fortress or an auxiliary fort it was often porticoed and had a large main gate. They all typically had three distinct sections.

A courtyard:

  • The interior courtyard was paved or gravelled with a roofed colonnade that ran around the sides. The roofed areas sheltered offices (used as scholae, etc.) and storage facilities (for the unit’s armamentaria).

A cross hall (basilica):

  • Directly across from the entry, on the far side of the courtyard, was a basilica. It stretched the entire width of the principia and was typically the tallest structure in a camp. Internally it was divided by columns into three parts comprised of a two/three storey nave flanked by aisles (each 1/3 the width of the nave). Light spilled into the interior from rows of clerestory (clear storey) windows that pierced the upper walls of the nave. On important occasions the cross hall would accommodate an assembly of (the majority of) the unit’s troops for an address by the commanding officer. A raised platform known as a tribunal was located at one end of the nave (sometimes at both ends) for the CO to administer justice, issue orders or speak to his men. An area would have been designated as an auguratorium for the conduct of religious ceremonies inclusive of sacrifices and auspices (augeries).

An array of rooms to the rear:

The rear typically housed five rooms.

  • The central one, the sacellum (or shrine), accommodated the chapel which housed the legionary standards and the treasury. It was usually a stone structure, even in forts that were still in their timber phase. There was often an area for witnesses to observe ceremonies held within through a grille near the imperial altar. In cohortal forts it held the Imperial and (smaller) century altars. Altars were replaced annually, and the old ones were buried outside (i.e., in the parade square) with reverence.
  • Off to one side was the tabularium legionis, the pay and records offices of the unit. Service, promotion, decoration, transfer and secondment records along with duty rosters, leave schedules, and illness reports were maintained here. The records were inscribed on a variety of media, depending on local availability; slate, wooden waxed tablets, ink on papyrus, etc. The tabularium was overseen by the cornicularius, who was assisted in his administrative duties by his deputy the actuarius and a number of librarii.
  • The offices of the signiferi were also located here.
  • The CO’s office (officium) was typically situated at one end with easy access to the tribunal.
  • The administrative office of the legionary cavalry was known as the tabularium equitum.

The tabularium legionis should not be confused with the Tabularium, the public repository for state records, located in the Forum Romanum.