One of the things that would make siege battles more interesting, would be the inclusion of a wider variety of seige engines. Aside from the standard ladders, towers, and rams etc. they should include sappers, (like in the original Rome) ship-mounted siege towers, (for naval assaults) "fire raisers", (for burning down gates, like in the siege at Delium 424 B.C.) Sambuca style siege ladders, and Heliopolis type towers. Each should cost money, and should only be available to generals with the right skills. These are all described in "Classical Warfare in the Ancient World" by John Warry. I also think that heavy seige engines, such as onagers, should only be build able for sieges.I always hated it in Medieval, when I fought an open battle, and half my enemy army was trebuchets. Realistically who is going to drag a 500 lb. onager halfway across western Europe? Lighter Ballista are another story.