[Taqwus]

If I've the gems and mages, I like softening up the defenders with a wave of Seeking Arrows or (often more lethal, but harder to get) Earth Attacks or (N/A for astrals) Mind Hunts. Kill as many leaders first. I might also use Leprosy the turn I move the troops in.
In battle, fliers are good. Long-range / high-precision magic is good -- Soul Slay, Paralyze, mind bLasts, that sort of thing. Against missile towers, air shields are nice. Magic that hits a small area is good because the enemy will probably be bunching up near the gate. Tramplers, high-fear units and fear magic can be used to clear a path. Howl and artifacts which cause it are good; the wolves appear frequently inside the walls; very good for absorbing tower fire, enemy spells; can cause your own mages to waste gems protecting them 'tho.
It might be a good idea to use cheap mindless arrow fodder (eg longdead, mannikins, dispossed spirits) on the front flanks (hold orders) to absorb tower fire while your more valued units go for the gate; haven't tried that yet.