Hey Omanes, good to see you made it over here! No worries about asking questions, regardless of how stupid you may think they are. That's partially what the rest of us are here for, after all.

cegman is correct, though. Princesses need not fear border forts. In fact, the only agents that do need to worry about passing BF's are spies and assassins. All other agents -- princesses, emissaries, imams, inquisitors, etc. -- can pass freely between provinces without hinderance.