I think the civilized units often have lower defence skill not because the armor compensates for it, but because the fact that they are wearing armor decreases their ability to defend themselves.
It is much easier for an unarmored man not to be hit, while an armored man can't move as fast and has to rely on the armor: Lugoae dodge blows that a Babylonian Spearman has to take, without being better fighters as such.
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