Help with F '14 Liege

[Lohengrin]

Thanks for the advice, Ed. I kept getting the feeling I was doing something wrong by not assaulting the forts properly, or something. In a prior play, I did manage to disrupt a fort MG unit, but it rallied the next turn. MG and gun units are very hard to disrupt normally (due to low density I surmise), so I cringe at the prospects of gaining that result against them in a fortress.

As a matter of curiosity, how would the other nations without the super-heavy siege artillery hope to take fortresses like, say, the French and Metz? Would they have to actually lay siege and starve them out? It calls into question the whole fortress doctrine and its apparent abandonment from the levels seen in WWI (Maginot notwithstanding), given that they still posed a serious military obstacle.