I am talking from a Union perspective, but I did capture a seige cannon from the CSA so I know they have them.
I noticed when I stacked two seige cannon (I am talking about those big guns buildable only in NY and PA) in a stack seiging a fortress, it appearred that I reduced the effectiveness of a fort MUCH quicker. ((ie changed beseiged icon in the tile to say something to the effect that the fort no longer gives any benefit.)
So I am assuming these big guns do a number on forts.
Assuming my observation is correct, how do you implement them? Just drop a couple of guns in the stack or do you use them in some sort of divisional configuration? If you do use them in a divisional what is that configuration?
I did a search on seige cannons and didn't find anything, so my apologises if this has been brought up before.