There are only 2 parameters affecting the Damage dealt during an attack:
- Offense of the attacker
- Defense of the defender
The Damage is calculated simply by reducing the Offense by the percentage value of the Defense (rounded down).
All the Zombies have a Defense value of 0, meaning they don't reduce their received damage at all.
In short: Each armor point equals 1% damage reduction.
25 armor = 25% reduction -> 75% damage taken
50 armor = 50% reduction -> 50% damage taken