All Roman forts followed a basic plan that can crudely be expressed as a central range of administrative buildings that included a pair of granaries or store buildings (horrea), a headquarters building (principia), and a house for the commanding officer (praetorium). The front and rear portions of the fort would then contain barrack buildings and stables, whilst latrines, ovens, and workshops and stores were built into the back of the defences.
The evidence clearly indicates that there were changes made to the forts whilst they were under construction. Some were constructed astride the curtain wall, so that it often had to be demolished where it already existed and additional pairs of gateways had to be added so that there were an equal number north and south of the curtain wall. Others, probably the latest, were just placed up against it (as at Great Chesters, Housesteads, and Carrawburgh).