First of all, whats up with the triple post, man? Maybe you need to lay off the caffeine
But that said, I'm no battle expert but I know that the trick to seiges is just getting your foot in the door, literally. In most cases your seiging force is more powerful, but they cannot bring that power to bear because the gate is too narrow and the defenders block it.
Some tricks that I use (particularly vs the AI - I only have limited exp vs humans so far) is to put a fast, powerful unit at the front of my army. Like some heavy cav or something which can run in and breach the narrow gate portion and hold until the bulk of the infantry can pour through.
Additionally flyers work very well in delaying the defenders from being able to plug up the gate until your inf can arrive. Trampling units would probably be exceptionally useful as they could just wade through the defending stack of troops. And if you dont have any of that, just put your heaviest hitters right up front so they can carve a way through before you are torn to bits.
I'm also assuming that you are using your priests and wizards to good effect to begin with?