| The guy from that armor & horseback video is Mike Loades on his show "Weapons that made Britain"- great show!
One of the best points he made in that series was this: The armor of the day was well designed to protect the wearer from the weapons of the day.
The second best point he made was that no armor was perfect- each had a flaw that an experienced warrior with the right tool could exploit.
The main flaw, common to all armors, is the human inside- specifically his soft tissues (esp. the brain). While the armor may prevent a blade from slashing flesh, a spear from thrusting deep, or a mace from crushing a limb, the non-compressibility of the water that makes up so much of our bodies means a solid blow may result in hydrostatic shock that could debilitate a seemingly uninjured combatant. That is why maces were so popular after the advent of full plate. Ditto the development of the Mortschlag- the Murder Blow- where the combatant grasped the blade and struck at his foe's head using the crosspiece of his blade like a hammer or pick.
While a mace blow or mortschlag might not break bone or even cause a bruise, the resultant hydrostatic shock could disrupt the nervous system of the person struck, resulting in a "dead arm" or even a concussion- either of which is debilitating enough to leave the victim vulnerable to more lethal attacks. | 
And yet another word from our sponsors
