Help with F '14 Liege

[Lohengrin]

To those of you with some experience with the subject scenario, how do you win this as the German? Without siege guns, I bash my head against the fortresses and don't make a dent. I read that Ludendorff passed between the forts undetected at night, but this isn't possible due to Belgian defenders and the Night Rule. Even the 210mm artillery won't seriously damage units in the fortresses, so is the idea to ignore them? The best odds I've been able to get against one of the weaker forts on the East side is 1:1, which fails to capture the fort. Is there any hope of ever taking one of these without siege artillery?

I'm having a lot of difficulty with this and would appreciate some help. Thanks.