Min(imum) Damage is always inflicted, regardless of Armor, whereas Normal Damage can be reduced by it. Thus a fully upgraded Spearman (9 Minimum Damage and 5 Armor) always inflicts 9 damage even on a fully upgraded Footman (6 Armor), whereas the Footman's damage is reduced from 14 (1 Minimum and 13 Normal Damage) to 9 (14-5) against the Spearman.
Catapults always inflict 255 damage with a direct hit, and 63 (1/4) damage to any units in the eight adjacent squares (structures are only hit once, but adjacent units may still be damaged depending on where the structure is hit).
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