Olympia: The Age of Gods PBEM (Oct 4 2000)
Border control for provinces is the same as border control for sublocations (see Permission to Enter), with some exceptions.
First, the owner of a province is the first noble in the castle to which the province's garrison is pledged.
Second, the owner of a province also controls any city within the province.
Third, in case of attack, the province and the city are defended by the garrison, not by the first unit in the location.
Fourth, the garrison will not patrol the border (leaving it open) unless there is a noble seated in the controlling castle.