Some units wear a shield. A shield helps protect the wearer vs both melee and ranged attacks. All shields have a Parry value and a Protection value. The Parry value determines how easy it is to parry attacks with the shield. The Protection value indicates how much Protection the shield offers on a succesful parry. How it affects the wearer depends on the type of attack.
In melee combat a shield adds extra Protection to any defender who succesfully parries a melee attack.
Attack roll: Weapon's Attack value
+ Attack modifiers, if any
+ DRN
Defense roll: Defender's Defense value
+ defense modifiers, if any
+ DRN
+Shield Parry rating
If Attack roll > Defense roll + Parry rating ⇒ Attack hits as normal
If Defense roll + Parry >= Attack roll > Defense roll ⇒ Attack is Parried
If Attack roll =< Defense roll ⇒ Attack misses
When an attack is Parried the shield's Protection is subtracted from the damage roll.
In ranged combat a shield can protect against incoming damage.
Attack roll: Size points in the square
+2 if magic weapon
+ DRN
If Attack roll > Defense roll + Parry rating ⇒ Attack hits as normal
If Defense roll + Parry >= Attack roll > Defense roll ⇒ Attack is Parried
If Attack roll =< Defense roll ⇒ Attack misses
When an attack is Parried the shield's Protection is subtracted from the damage roll.
Shield hits can damage or destroy the shield. A shield has a Resistance, which is the same as its shield protection value, +5 if the shield is a magic one. The attack has a Break value, which is equal to the damage of the attack before any Protection.
Note that many spells have the Ignores Shields tag which basically means that shields will have no influence at all against that particular attack.
Break value: Damage of attack before reduction
Shield Resistance Shield Protection value +5
If the Break value is >= 3x the Shield Resistance, the shield is damaged (-20% Prot).
If the Break value is >= 5x the Shield Resistance, the shield is broken (-50% Prot).
An already damaged shield that is damaged again has a 25% chance of being broken.
Damaged shield protection values round down: that is to say that a damaged Scutata Volturnus offers 16 Protection.
A magic shield (a real, equipable one) that is damaged will repair itself after the battle. If a magic shield is broken, it will be permanently destroyed and the commander will revert to his or her standard shield if he or she had one. Damaged or broken mundane shields will be repaired when they are in a province with enough spare production resources.