Attack Types

Normal (most melee units)
Normal attacks do extra damage against Medium armor, and reduced damage to Fortified armor.

Piercing (most ranged attackers)
Piercing attacks do extra damage to Unarmored units and Light armor, and reduced damage to Fortified, Medium armor, and Heroes.

Siege (artillery)
Siege attacks do extra damage to Fortified armor and Unarmored units, and reduced damage to Medium armor and Heroes.

Chaos (A few Creeps, Powerful Summoned Units)
Chaos attacks do full damage to all armor types.

Magic Damage (most spellcasters, air units)
Magic attacks do extra damage against Light and Heavy armor, and reduced damage to Medium, Fortified armor, and Heroes. Magic attacks do 66% extra damage to ethereal units, and zero damage to magic-immune units.

Hero
Hero attacks do reduced damage to Fortified armor.

Normal
Missile
Artillery
Instant

Armor Types

Unarmored (most spellcasters)
Unarmored takes extra damage from Piercing, and Siege attacks. Some unarmored units such as Huntress, Gargoyle, Hippogryph, and Druid of the Talon in Storm Crow form have armor for balance reasons. Most unarmored units do not have armor.

Light (most flying units)
Light armor takes extra damage from Piercing and Magic attacks.

Medium (most ranged attackers and workers)
Medium armor takes extra damage from Normal attacks, reduces damage from Piercing, Magic, and Siege attacks.

Heavy (most low and high tech melee units)
Heavy armor takes extra damage from Magic attacks.

Hero
Heroes take reduced damage from Piercing, Magic, Spell, and Siege attacks.

Fortified (buildings)
Fortified armor greatly reduces Piercing, Magic, Hero, and Normal attacks, but takes extra damage from Siege attacks.

Damage and Armor Class System
Every unit in the game has an armor type and attack type (like StarCraft).

Each attack type is better or worse versus other armor types. For example, a Grunt has a Normal attack, which does 150% damage versus "Medium" armor units like the Archer, Rifleman, Troll Headhunter, and Crypt Fiend. The Archer has a Pierce attack, which does 100% extra damage versus "Light" armor units like the Gryphon Rider.

This system is in place to encourage unit counters and unit mixing in combat. If the opposing player builds ranged attackers, then the natural counter would be to build melee units, which have an attack bonus versus them. However, since ranged attackers have a high damage, you would benefit by building a mixed group of melee and ranged attackers since your ranged attackers will out-damage the opposing melee units if they are protected from direct melee damage themselves.

Warcraft III: The Frozen Throne: Attack Type vs. Armor Type Chart

Light Medium Heavy Fort Hero Unarmored Normal 100% 150% 100% 70% 100% 100% Pierce 200% 75% 100% 35% 50% 150% Siege 100% 50% 100% 150% 50% 150% Magic 125% 75% 200% 35% 50% 100% Chaos 100% 100% 100% 100% 100% 100% Spells 100% 100% 100% 100% 70% 100% Hero 100% 100% 100% 50% 100% 100%

Warcraft III: Reign of Chaos v1.06: Attack Type vs. Armor Type Chart

Light Medium Heavy Fort Hero Unarmored Normal 150% 100% 100% 50% 100% 100% Pierce 75% 100% 150% 35% 50% 150% Siege 50% 100% 100% 150% 50% 150% Magic 100% 200% 100% 50% 50% 75% Chaos 100% 100% 100% 100% 100% 100% Spells 100% 100% 100% 100% 70% 100% Hero 100% 100% 100% 50% 100% 100%

Damage Reduction or Increases for Armor
For positive Armor, damage reduction =((armor)*0.06)/(1+0.06*(armor))
For negative Armor, it is damage increase = 2-0.94^(-armor) since you take more damage for negative armor scores.

Consult this chart* for more information. A Positive Armor of 1 reduces damage by about 5.7%. A negative armor of 10 increases damage by 46.1%. A unit with 20 armor basically has 55% extra hitpoints -- 100 would become effectively 155.

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