The idea of the leaping and dancing fighter is strictly a fantasy concoction. Armor just kept getting heavier and more advanced until the invention of the firearm rendered even the heaviest armor capable of being produced at the time essentially moot.
I beg to differ.
Please bear with me as I try to set straight some common misconceptions.
Unarmoured, highly mobile fighting (not "leaping and dancing" but you know what I mean) has been a very successful style for many centuries, even against armoured opponents. Few know this because most people concentrate their research on the late middle ages.
What we should also examine is warfare in Roman times and the migration period. Most importantly the battles between Germans and Romans. Make no mistake about weapon quality then, both sides had outstanding weapons that were at least as good as those of the late middle ages, sometimes (in the migration era) even leagues better.
Germanic warriors disliked armour, as not to say despised it. They had helmets and shields, but no body armour. The shields were very light and designed only as missile defense. A shield would simply shatter from one solid blow with a melee weapon. The helmet did have protective value, but was also richly ornamented as a status symbol.
Basically what the Germans did to survive a fight was not to get hit. Both sides were comparable in numbers and quality of training, but while the Romans preferred a more solid, defensive fighting style, the German directive was speed, speed, speed.
Their swords (Spathae) were extremely light (around 2 lbs at 40" length) and could be wielded at great speed. Any kind of heavy (melee-suited) shield or armour would have bogged the warriors down and negated the speed advantage.
We can imagine it like this: in the beginning of a battle, spears and arrows were exchanged. These were blocked with shields. Once the armies closed in for melee, the shields were either discarded or, at best, taken into melee to absorb a single blow. With the shield gone, we can assume that the Germanic warrior would draw his Seax as off-hand weapon, since the broadswords were strictly one handed weapons (very short hilt) and having one hand free does not make sense. The style and construction of a period sword belt (with the Seax right next to the sword) supports this assumption.
So, it did also surprise me when I learned this, but these blokes were actually unarmoured dual wielders, to put it in game terms.
[aside: note, however, that those rectangular Roman Scutums we know from contemporary movies etc. were in reality extremely scarce, and sort of "special forces" equipment. Depicting an entire legion armed with these would be like a modern war movie about a full division of Delta Force. Most Roman legionaries had round shields just like there adversaries.]
We know the Germanic tactics was successful because the Romans were not. The Roman empire expanded thousands of miles until ocean or desert set an end to their expansion, except in the north, where the border was practically at their doorstep, despite various attempts of expansion. Which means that the German losses can't have been worse than the Roman casualties.
So much for Chapter One. But it goes on:
The reason why armour kept getting heavier lies not in any kind of melee weapon. For centuries, a mail shirt (which the Germanic peoples adopted only at the end of the migration period) provided adequate protection against all weapons of the era and way into the high middle ages -- not total protection, but the best compromise between protection and mobility.
Then up came: the Crossbow, introduced by returning crusaders in the 12th century. Suddenly, any peasant could operate a weapon that could fell a professional warrior (read: knight) with a single shot. This was an unbearable situation for the nobility. The only way to counter the new weapon was, you guessed it, heavy armour.
So there you have it, the late medieval arms race was actually initiated by the crossbow (not the longbow, no matter what the English might say). The light, fast swords that had been so successful for over a thousand years were suddenly not able to penetrate the new armour anymore. So the swords got heavier, and other armour-piercing weapons were designed (like those we know from the Conan game).
While the armour kept getting heavier and thicker, but before the advent of the firearm, the swords changed their appearance once again, following the example of other anti-armour weaponry: relatively slender, rigid, long blades for piercing came up, after the previous attempt to cut through armour with heavy blades proved to be a dead end. These new swords were sort of the father of the rapier.
For completeness sake, armour was eventually abandoned indeed due to its ineffectiveness against firearms. From that point on, the piercing blades did not have to be so rigid anymore, and so the lighter, flexible rapier developed.
So, we end this broadcast and return to our regularly scheduled program. With all that I just meant to say: unarmoured fighting is NOT a fantasy concoction, especially not in the context of early middle ages.
Note to self: since I banned crossbows from my game, I should probably also ban plate armour.