This was the administrative and religious centre of the fort and was the most important building in any Roman fort. It was situated at the centre of the fort where the via praetoria and the via principalis crossed.
The Principia is laid out in a square with three wings around a parade ground, and a gate on the south side. The north wing consisted of three rooms.
The central room was the sacellum, the regimental shrine, which served as a focal point of the headquarters. The standards of the units were kept here.
In the centre of the room was a sunken pit which acted as a 'strong room' for the regimental pay chest. The money was kept here to deter thieves who would be committing sacrilege if they stole from the shrine. The standard bearers acted as treasurers of these funds keeping track of what money was paid in and out. The pit is the first example to be found in a temporary timber fort.
The rooms to either side of the sacellum were probably used by pay and records clerks.
The two side wings of the Principa building housed long rooms which were probably armouries.