Interaction with damage
Normal damage is reduced by armor. For each point of armor, the final damage of an attack will be lowered by one.
Piercing damage completely ignores armor. Negative armor still makes you take more damage.
Siege damage is not hindered in any way by the armor on a blocking unit, but will deal less damage to the tower if the tower itself has armor.
Artifact 2.0 Beta
Negative armor will cause the affected unit to take more damage than the attack would have done otherwise.