When could occur the situation where a roman coul have a empty fort and use it for the control ofa province if roman player is not allowed to leave a fort empty of troops ?
15.3 Diocesis Victory Points:
CONTROL: "For this purpose only, a Roman Faction player earns Roman Victory Points if he has at least one Legion in the Diocese and only Forts/ Fortresses in all the Diocese’s other Provinces. (Forts/Fortresses do not control a Province at any other time.)
8.2 Buildings Romans : "While building the Roman player may also pick up Legions from map, in order to use them in expansion. Areas may not be
vacated this way, one Legion or a Fort/Fortress must remain in each area.
So a fort can be left alone in an area during the Building Roman phase.