Nargott
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Well, that's the point – it only applies to the first three tiers.
In other words, the racial skill works immediately when breaking through.
And, at the same time, it provides some protection against rushes. When someone like Orson arrives early with a crowd of low-level skeletons/zombies.
Later on, you have a choice – either keep these three tiers or set more high-level units as enemies.