I've run a lot of AD&D sieges with fair amounts of magic. The main difference from historical norms - apart from the use of high level strike teams by both sides - is the battle for air supremacy. Generally you have Wizard 'bombers' whose goal is to fly over the enemy lines/castle, Invisible if possible, and Fireball the hell out of the enemy. Then you have Fighter 'interceptors, warrior types with Fly spells or winged mounts (also may be Invisible) whose goal is to kill those wizards. You end up with a rather 20th century battle where whoever wins air superiority by wiping out the enemy's flight-capable forces, wins. This is much more true of sieges than field battles because in a field battle the short time duration means that spellcasters may quickly run out of magic, limiting their impact. In a siege, spellcasters can always wait a day to 'recharge', plus you have static targets - the castle and the besiegers' lines.