Also a rule inherited from the boardgame regarding breakthroughs: you need to wait until the 2nd round of combat to see if you can select your future breakthrough place. This is a mechanic currently under debate at the beta forum, but you did not hear it from me.
As for forts: yep, damn hard business. What I do is really simple: when the siege window comes up, I first attempt to fire the artillery on it, then "besiege operations" (having the game check for its fall), then delay siege. I do not assault unless speed is absolutely essential, and it rarely is. Assaulting is VERY bloody, especially if you have no artillery support.
I think at start, there are two forts to consider assaulting: Belgrade, and Liege. I think it does not worth it for Liege (and you have a good chance to have it go in August, due to your Austrian siege arty), and Belgrade, well, it depends on my mood. The Serbs can get quite an annoyance if not contained early on, but because of the assault you may end up not having enough units at hand to contain them with.