Protection is a property of units that reduces damage that they take from most attacks. It is generally provided from either manufactured armor or from natural protection (tough skin, scales, being made of stone, etc.) If a unit has both manufactured armor and natural protection, their values stack with diminishing returns.
Natural and armor protection combine according to the following formula:
total_prot = natural + armor - (natural*armor/40)
Since manufactured armor provides protection, the increasing technology level in Middle and Late Age means that most national and independent troops tend toward higher and higher protection values in later ages.
Whenever a unit is struck by something that might cause damage, the game calculates the damage dealt as follows:
final_damage = (attack_damage + DRN) - (protection + DRN)
where a final damage roll of zero or less doesn't do any damage at all.
Some attacks deal piercing damage; these attacks count only 80% of the target's protection.
Some attacks are marked as armor piercing (AP); these attacks count only 50% of the target's protection (stacking with piercing damage, if applicable).
Some attacks are marked as armor negating (AN); these ignore the target's protection.
Note that magical attacks are not AN unless specifically noted. Typically fire attacks are AP; typically lightning attacks are AN.
Protection is tracked separately for the head and body of units. Natural protection protects both locations; manufactured armor counts separately for each. For instance, chain mail protects the body but not the head, and a helmet protects the head but not the body.
Shields also have a protection value. If a unit has a shield, any hit to the shield results in the shield's protection value being added (without diminishing returns). In most cases this results in a final protection value high enough to completely negate the damage dealt, but not always.
Protection amounts to a flat subtraction from the damage dealt by most attacks. High protection values can thus make a unit nearly immune to weak attacks, requiring a lucky DRN role to overcome; units with low protection values are vulnerable to large numbers of weak attacks, particularly arrows.
Various spells can boost units' protection; others can help you overcome targets with high protection.
The spell Legions of Steel (Construction 3; Earth 3, 40 fatigue) increases the protection value of the armor worn by a large group of troops by 3.
Several spells increase the base natural protection of targets, but make them more vulnerable to certain sorts of magic:
One option for dealing with high protection units is to simply hit them harder; many nations have or can summon troops with two-handed weapons who are good at this. Many forged weapons provide very high damage values and/or AP or AN attacks; these can be useful in breaking through exceptionally high protection values on immobile pretender gods and some supercombatants.
The spell Strength of Giants (Enchantment 3; Earth 3, 40 fatigue) grants +4 strength to a large number of troops, which can make an enormous difference in their ability to crack through well-armored enemies.
The spell Weapons of Sharpness (Construction 7; Earth 5, 20 fatigue) grants a large number of troops armor-piercing attacks. This is extremely effective against well-armored targets.
The spell Destruction (Alteration 4; Earth 3, 40 fatigue) destroys nonmagical armor of a large number of troops.
Troops with heavy armor tend to accumulate fatigue in extended engagements, which makes them more vulnerable to critical hits. Swarming them with summoned skeletons or bugs from Swarm will eventually fatigue them out.
Finally, many spells bypass armor partially or totally. Air evocations can do large amounts of AN damage, and Death and Astral spells like Shadow Blast and Soul Slay ignore armor but allow for a Magic Resistance check.