Combat

Combat is the art of using units to kill or rout enemy units during battle. This article concerns mainly melee attack, melee defence, the chance to hit and damage dealt. For some information on resistances, see Damage channel as of now.

Chance to Hit

The chance to hit is determined by the following calculation (attacker's attack skill vs. target's melee defense):

Chance to hit = ( 35 + ( Melee attack - Melee defense) ) %

Each unit has a minimum 8% chance to hit after applying modifiers and a maximum of 90% (or 10% chance to miss). Stats are modified by fatigue levels before the calculation.

Example

Temple Guards		fight	Rat Ogres
	Melee attack 32 Melee defense 38	Lizard chance to hit → 35 + (32 - 26) = 41% ← Rat chance to hit 35 + (26 - 38) = 23%	Melee attack 26 Melee defense 26

In short

A higher attack skill makes units more likely to hit, while a higher defense makes them more difficult for the enemy to hit. However, since pure random number generation is used, it's entirely possible to have a string of hits or misses. Higher skills levels merely reduce the chance of that occurring.

Damage

If the melee attack of one unit hits another, damage is dealt.

• The base damage value is fixed according to the weapon equipped:

Total damage = Base Weapon Damage + Armour-piercing damage

• Units charging the enemy will also deal impact damage, calculated according to their speed and mass. The resulting damage is 70% armour piercing. See Patch 1.12.1 for more details. In addition, Charge Bonus is applied to attacks for a short period of time (15 seconds) after chargin
• Armour reduces incoming base damage, but not armour-piercing damage. The armour value corresponds directly to a randomized percentage-based reduction, where the minimum value of damage reduction is half the armour value, and the maximum value of damage reduction is the full armour value:

Mitigated Damage Taken = Base Damage * ( 100 - Armour * random(0.5, 1) )%

NOTE: the "Armour*random(0.5, 1)" part of the equation is capped at 100 value, to prevent negative values. This way, the damage reduction increases linearly up to 100 armour. After that, you get diminished returns for every point of additional armour. That is to say, the absolute value of damage reduction is diminished, but due to the subtractive nature of it the effective health of the unit increases exponentially the closer you are to 200 armour. For more details, see the final paragraph of Armour.

• Shields provide a flat percentage chance to deflect missiles, but do not affect armour values.

Example

Temple Guards		fight	Rat Ogres
	Damage 13 Armour-piercing damage 29 Armour 85	Lizard minimum damage → 13 * (1 - 30*0.01)) + 29 = 38.1 Lizard maximum damage → (13 * (1 - 30*0.005)) + 29 = 40.05 ← Rat minimum damage (22 * (1 - 85*0.01) + 58 = 61.3 ← Rat maximum damage (22 * (1 - 85*0.005) + 58 = 70.65	Damage 22 Armour-piercing damage 58 Armour 30

• A word of note: Attack animations matter. Some units have very favourable or unfavourable attack animations, so the pure stats are not always all that matter.

Takeaway

• Melee attack and defense are responsible for the chance to hit and increasing them according to the units' purpose (more defense for tanks, meat shields, and tar pits, more attack for those who are supposed to take down the enemy) is generally a good idea.
• Armour is essentially capped at 200, providing the maximum (100%) damage reduction against regular attacks. To bring down a unit with an armour value at or approaching this level, Armour-piercing damage is necessary. However, every attack in the game deals some amount of armour-piercing damage.

Sources

Parts of this article are possible thanks to ferriswheel9ndam9 on Reddit. (Some information is possibly outdated or incorrect, particularly concerning attack intervals).
For a very good summary of the theory behind combat see also this reddit comment.