@GrimGriefer No offense, but WTF.
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...knights in full plate armor. They mostly never die in actual battle before the invention of the longbow (which pierces armor)' - Longbow existed way before full plate armour. It's the arrows with bodkin point that pierces armour, not because of the longbow. At most, an arrow is only going to pierce through armour by a few cm, and underneath the armour, there's still chainmail or padded armour.
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Even the gaps in their armor had chain mail protection...' - That is usually found during the transitional armour period (chainmail with plates to full plate armour) and aiming for the gaps are one of the few ways that can injure the knight. In other words, it can penetrate... that's the reason swords and the like became thinner and pointier, but the gaps are small, and it's easier to use a mace or something, which is also why pole-arms, warhammer, etc. were the primary weapons to begin with.
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Knights in FULL plate armor couldn't move properly...' - There's youtube, please go see some videos of people wearing PROPER full plate armour. (Not the kind of crap documentary you find on History channel.) Or, if you can't be bothered, here's the answer: they can move just fine.
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Also, knights in full plate armor before the longbow didn't use shields...' - I don't even... ah, I get it, all this must be an april fool's joke.