One thing to remember is that we're comparing 16th and 17th century Japanese technology with 12th - early 15th century European in Medieveal:TW. You wonder what European armorers and swordsmiths would have accomplished if they had another 2-3 centuries to develop their craft. Instead, the development of firearms pretty much made even the best armor increasingly obsolete after 1450 or so (one reason for the cut-off date in MTW). By the 17th century there was no point in heavy armor for combat, since the musket ball could easily pierce it. Cavalry became lighter and faster, depending on shock action to overrun infantry and gunners before they could get off many rounds. That led to the use of pikemen to hold off the cavalry and their integration with musketeers (beginning with the Swiss system), until ultimately the invention of the bayonet made the use of pikemen obsolete (although they turned up in odd places even during the Napoleonic wars - as with the Moscow militia pikemen who played a key role during the battle of Borodino in 1812, actually just by standing there).