[Fantomen]

Flyers are good, flies over the walls. Tramplers or thugs or other units that dish out lots of damage quickly are good to get you through the gate. Making the enemy rout works as well, with spells like panic or terror.

If you are uncertain of storming successfully Just staying on siege may weaken the defenders if they have supply problems, or on a grander scale if that fort is important for recruitment or income. Like sieging a capital can be strategically sound even if you are unable to breach the walls, just to deny capital only mage recruitment and gold.

On the flip side mages inside a sieged fort can still cast summoning rituals and the like (if there is a lab), so the defending army may very well grow stronger during the siege.