I find it interesting that a few roles like the Praefectus Castrorum seem to hint at some divisions (garrisons especially) of the army not actually comprised into one of the Legions proper. While a legion could be stationned at a Castrum, the administrative force of the Castrum proper looks not to be part of a Legion itself, a legion being mobile by definition.
Balmung:Â Suen Shyu