The armor specialty provides 5% of the percentage value of the armor skill (this skill adds nothing to defense; it simply reduces damage dealt to units).
For example, with an expert "armor" skill:
At hero level 5 it will be: (15%+15%x5x5%)=18.75% damage reduction.
At hero level 10 it will be: (15%+15%x10x5%)=22.5% damage reduction.
At hero level 20 it will be: (15%+15%x20x5%)=30% damage reduction.
And so on...
Added after 55 seconds
