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RANGED ATTACKS
A unit's defense does not matter when being attacked by an arrow. Precision just determines which square the projectile will hit.
The attack roll on the projectile is 10+2d6,
while the victim's roll is 10+shield defense modifier+2d6.
A tower shield for example has a defense modifier of 4.
It is not clear, though, if the missile uses area attack code or not. Most likely, it seems, it chooses one random unit in the hit square (not necessarily the target square) and rolls for a hit against that unit.
The higher the precision, the smaller the statistical deviation from the target coordinates. Deviation growth with distance to target o.c.
With very low prec, sometimes spells fan out all over the field when the rolled deviation number is very unfavourable.