Enfid wrote:
I would say the walls are either enchanted by the opposing spellcasters to be resistant to magic or gives protection to those on it, or has holy/unholy spirits guarding it that magic has no/little effect, or the defenders have a collective willpower enough to make spell casting futile. Whether this means magic is automatically dispelled or every unit on walls may take a 2+ Will of Iron roll requires some playtesting.
In concept I agree. In order to limit magic's effect during a siege some basic rule that either does not allow spells to be case to/from elevated positions or else gives an automatic 2+ or 3+ Will of Iron (good thought) in such cases. This could still allow for winged Nazgul, for example, to fly over and nail the forces on the other side but I guess this would put them at risk and may not be done much. Of course it also matches the terror and effect that the Nine had swooping down on Minas Tirith during the siege there. Either way though, it would limit or prevent spell users both on the walls and on the ground from casting at each other. If you want to use spells you need to either breech the walls or scale them. Once on the same level or side as your opponent then it's back to normal.
I do have to say though that most of your specific suggestions for explaining it aren't really in line with how I see magic and such things in Middle Earth (but then, not all of what GW has done in WotR magic is the way I see it anyway). I think just a basic rule without trying to come up with a ME "theme" justification is fine. After all, we're just trying to find some house-rule options to make siege play in WotR more practical and enjoyable and not trying to simulate something that's Tolkien-specific in the WotR system.