Where possible, it's siege towers all the way with me. Other methods seem to take very high casualties from the shooting of defending towers. The best part is taking over the defending towers on the walls and having them automatically rain missiles into any defenders down below. With large stone walls, the firing from siege towers is a final great bonus.

But I do tend to play Romans, whose infantry excel in fighting on the walls.

I hope MTW2 refines all this, as I think siege towers are currently too much of a no-brainer.