The Heavy Knight is the Fast Unit of Late Middle Ages. Having moderate damage and defense point, and a special ability of "Chivalry", Heavy Knight gains bonus damage and defense points of +5 when it is the only unit with "Chivalry" ability of your army. Effective against ranged unit but vulnerable against light units.
During Late Middle Ages, Heavy Knight is comparable to Great Sword Warrior and Imperial Guard with similar stat points. Players should choose their units depending on enemies' unit type more than stats.
The Heavy Knight is an improved version of the Knight of the previous age. It gains a lot of defense and a moderate amount of attack power. It retains the speed of the Knight. In this age, the Great Sword Warrior has the same amount of defense and 3 more attack power than the Heavy Knight (without chivalry), making Great Sword Warriors a better overall choice than Heavy Knights. While Heavy Knights have an attack bonus against ranged units, ranged units generally are two shotted by all units except for artillery and heavy units, and therefore this bonus is insignificant.
If used, it should be used sparingly, with the best idea using one per battle to take advantage of chivalry. Otherwise, Great Sword Warriors should be used in replacement of Heavy Knights.
During the 100 years war against the English, the French were heavily relying on heavy armored, mounted knights and men-at-arms. Heavy Knights were often equipped with some form of scale, silver, chain mail or lamellar armor. The extra armor made them very resilient and strong into close combat, but vulnerable at range. The horse has to be very strong to carry the armored rider, his own armor, and at the same time charging the enemy. Despite the fact that they were the strongest cavalry in the 100 years war, they weren't a match against the English longbowmen. In general, the knight was able to easily charge and kill ranged units, but the English bows had enough power to go through the knight's armor.