We discussed this topic in a recent Saturday Showcase. I talk about armour penetration here. When we think about defeating fully armored men in battle, it is important to remember that not all defeats require penetration their armour. Fully armored men can be overpowered and overwhelmed. Groups if armored men can be out maneuvered and routed. In this way Armoured men can be, individually, very hard to kill, and collectively beatable at the same time. That is not to say armour penetration does not exist. It did, and of course there were even more ways of going around armour than through it. Armour penetration, rather than allowing knights to be mown down en masse, added to the moral and tactical effects if something like archery, as u/Hergrim discusses this here.