Tactical combat mechanics?

yeah, counter base value is less than 50.  I think it's 25%.  first counter perk adds 25% to that, so you hit at 50%, next one boosts it to 75% (total).

N/A

I suspect that counterattacks deal damage computed using half the attack score of the unit making the counterattack, rather than by halving the damage which results from the full attack score. In the case of a counterattack against a zero defense target, this results in half damage. Against a target with some amount of defense, the reduction in damage can be much greater.

Regardless, the XML file CoreUnitStats.XML includes a multiplier for counterattacks, which is listed as defaulting to 0.5. Thus, the percentage reduction for anything that lacks a counterattack damage multiplier should be 50%, though this is not necessarily a 50% reduction in damage (again, I suspect the multiplier is applied to the attack score rather than to the damage).

In case you don't want to read what Primal_Savage linked you to, the swarm bonus is not a percentage bonus but rather an additive attack and accuracy bonus (1 attack and 5 accuracy per friendly unit adjacent to the target). Thus, the percentage damage bonus granted by swarming an enemy is highly variable, depending on what your units' attack scores look like, on what your target's defense score looks like, and on how likely your units are to hit their target.

Ah that makes sense,

So, counter attack: 25% by default (with sword)

Warrior perk 1 50%

Warrior perk 2 75%

If there was a warrior perk 3, then it'd be 100%

Is it really 25%?  

<UnitStatType InternalName="UnitStat_CounterattackMultiplier">
        <DisplayName>Vengeance</DisplayName>
        <DisplayNameShort>VNG</DisplayNameShort>
        <Description>Unit deals extra damage upon successful counterattack.</Description>
        <Icon>Piercing_Stat_Icon.png</Icon>
        <Hidden>1</Hidden>
        <DefaultValue>0.5</DefaultValue>
        <UnitStatGrouping>CombatStat</UnitStatGrouping>
    </UnitStatType>

This implies that it is 50?

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Yep - thanks. That makes sense.

So that means that warrior gets 100% of the damage with their full ability then.