There are three types of armour:

shield, which works at all times but only works to the front and left.

armour, which works at all times but is susceptible to AP weapons.

defense skill, which only works in close-combat and is susceptible to ranged attacks.

The armour value includes many different things, including helmets. This is probably where the armour value is coming from.

The shield value not only represents the size and shape of the shield but also how trained the unit is at using it.

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