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Regarding the Guardians' ability – it's a good idea, I had forgotten about it.
The Fire and Lightning Shot looks interesting, but it would be better to turn the ballista into a combat magical mechanism, similar to the dwarven cannon or their runic stone.
Regarding the effects:
If we take the dwarf model as a base and use the ballista effect, the dwarf will be in the center by default, and the ballista will extend beyond the boundaries of the creature's cell, which is not ideal.
Regarding the cannon: the Askanian firearm should remain at a low level of development, and I would like it to stay that way. I was devising new ammunition for the grenadiers, not muskets, precisely for this reason.
Despite all of the above, I welcome cannons based on magical power; it's interesting, and powerful magical cannons are not common.
I liked the idea of lightning; such a machine could be made similar to the Air Tower from MMDOC, a stone floating in the air surrounded by a whirlwind, or we could go another way and create a machine similar to the Polish one, with a stone attached to the ballista. Another ballista could be made with explosive projectiles