re outflanking the forts (and even pillboxes) - a lot of forts were built with almost no "forward" lines of fire, but massive concrete walls/earthworks facing forwards.
The main armament was facing left/right-forwards, so that attacking units would be caught in crossfire from their flanks, and due to overlapping fields of fire, if you turned to flank them, you'd be under fire from front and back.
The area in "front" of the forts was meant to be saturated by indirect artillery, and some cover provided by smaller anti-infantry pillboxes. I'd recommend going and seeing one for yourself, it's quite interesting
As for damage, I saw one of the Czech forts (Dobrosov), where Germans were testing special bunker-buster ammunition Röchling. Despite firing point-blank (200m direct fire), they didn't manage to break through the wall (it's about half-way through destroyed) - remains of one of them is still embedded in the concerete wall.
The only internal damage was from one hit via the removed fire-slit. For comparison, in Belgium Röchling managed to penetrate 3.5m (~12') concrete walls.
That said, I think it would be singularly unpleasant experience being inside during a bombardment like this, and I doubt anyone would be very efficient in doing anything even a few minutes after it was finished.
V.