Map command/terrain suggestion

[Twan]

Would it be possible to add a map command or terrain type making impossible to build forts on a done province ?

I think it would add a lot to the game if some provinces can't be fortified, considering how it's hard to take a fort in endgame it would allow to make far more interesting maps for big games, where key provinces/chokepoints change hands often even in late game ; actually the ability to cast first for the defender + a fort to be sure to be attacked only one time per turn is a so huge advantage it's easy to stalemate a province.