I think part of the problem is that fortifications like the Maginot line are really on a different niveau than standard fortifications as represented by the land fort in this game. I think the problem could be fixed by requiring a minimum level of forts and infrastructure before constructing a new very expensive super fortification, somewhat comparable to a super heavy tank compared to a heavy tank or super heavy battleship to a battleship. This would more accurately represent the maginot and help address the the problem introduced by impassable areas.