The Principia or headquarters building would have been the most important building for the Romans. This was where all the official documents would have been kept, the armoury may have been here, it's where the high-ranking officals would have had their offices. It is also where the soldiers pay was kept in a room called the strong room, which was sunken into the floor and had double thick walls. Just outside of the strong room would have been the chapel of the standards, where the units standards and a image of the current Emperor would have sat. These would all have need strong protection and thus the headquarters is usually built first and built into the middle of the fort so it is protected on all sides.