Because the ground sloped from north to south at Housesteads, a terrace had to be built to hold the Principia and, for that reason, it is somewhat smaller (ca. 632 square metres) than those found in comparable forts. The main entrance was on the east, in line with the via praetoria, and there was a smaller door leading from the north into the cross hall. A large fragment of a relief carving of the god Mars was found nearby—probably one of a pair that decorated the main gate, similar to the arrangement above the East Gate of the fort.

The courtyard was substantially remodelled in later times but there was clearly a colonnade on three of its sides when it was first built. A pair of doors in the middle of the west wall lead into the basilica. The nave was higher than the aisles on either side and probably had a pitched roof. The interior was lit by clerestory windows set high in the walls. It served as an assembly hall and had a raised platform or tribunal at the northern end where the commanding officer could stand to address the troops.

A set of five rooms occupied the western aisle, centred on the aedes where the regimental standards—sacred objects to the legionaries—were kept, along with a statue of the divine emperor and an altar to Jupiter. The aedes was separated from the nave by a screen made up of decorated stone slabs set into slots in the sill. None of the slabs have survived at Housesteads but they have at Vindolanda (right). Sockets had been cut into their tops to hold a metal grill. The legionaries queued up at the chapel to receive their pay. The regimental standard bearer (signifer) served as paymaster.

The rooms on either side of the chapel are usually interpreted as offices—one for the signifer and another for the cornicularius, the adjutant. Another room served as the strong room, where the regimental funds were kept.

Reconstruction of the Principia