Since responses seem to ebb away a bit, here is just my comment on your proposal:
Hmm, I agree to some extend, but one can also be of the opinion that these rituals take a full month to complete and that almost all gems are already consumed before the Last day. So the enemy mage just managed to finish his spell a couple of hours earlier. Or maybe the gems are used in advance to temporarily empower the mage's apprentices, which were killed by the flames as well?
The question is whether this problem is misbalancing or annoying enough to warrant the additional coding efforts. And I do not think so. After all, all my mages ought to be forging and casting deep down in subterran labs where they are 100% save from seeking arrows and fires from afar anyway, but this would only imbalance these spells compared to earth attack...
The game needs to abstract these mechanisms anyway, and I am personally happy with the way these things work currently.
(PS: Oh, please refrain from responses that only insist that CorpseConstruction must be cast on top of the highest spire of the castle in order to work, thus leaving the mage exceptionally vulnerable to SeekingArrow and FiresFromAfar!
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