Just found it there, and thought it's good to know this stuff for the battles:
Armor:
In general, fully effective against all attacks (at any direction), except:
- Half effective against armor-piercing missiles (crossbow, longbow, gunpowder)
- Half effective against armor-crushing melee (axes and maces)
Shield
Effectiveness depends on direction of attack:
- Against front: full effectiveness
- Against side (left and right): half effectiveness
- Against rear: zero effectiveness
Exceptions:
- Zero effectiveness against gunpowder (any direction)
Unlike armor, shields do not have decreased effectiveness against armor piercing missiles (crossbow, longbow) or armor-crushing melee (axe and mace)
So basically, against crossbows/longbows, shield is better than armor. However, against musketeers, armor is better than shield.
Shield is better than armor when fighting axemen.
Defense skill
Applies only to melee attacks from the front, but always at full effectiveness against melee at the front.
Total base defense is armor + shield + defense skill as modified by the direction and damage type
Each armor upgrade adds +2.5 (approximately) to the base armor amount. So a unit with 10 base armor with a gold (x3) armor upgrade has approximately 17.5 armor.
to bypass the shield, which is carried in the left hand?
The shield is equally effective for left and right. The myth that shields don't work on the right side comes from RTW, in which it really didn't work for the right side. However, the game engine was modified in M2TW such that shields work on left and right equally.
Furthermore, units with shields stats but have shields that are worn on the back (such as pavisse crossbowmen and Giltine's chosen) behave the same way. This means that the pavisse crossbow's shield has no effect against missiles that hit the unit from the rear even though the shield is graphically worn on the back.
Also it was discussed on Discord about shields:
Shield for missiles is 50% from both side and 0% from behind.
AP+Gun = -75% shield and -75% armour
Gun alone = -0% armour and -50% shield
AP alone = -50% shield and -50% armour