As new ways to wage war and inflict mass slaughter are embraced by armies all across the world, so too do the ways of the horse and sword change. In the place of the heavier Demi-Lancers arises a new breed of soldier: the Cuirassier. He retains most of his stats, but differs from the Demi-Lancer with several regards: it has more attack, better hitpoints, a better rate of attack and a slightly better move and production speed in comparison to his more ancient forbears, offering the French, Mexican, and German factions a powerful shock force for the mid-game in Rise of the Moderns. The problem here however is that once you obtain them, Cuirassiers usually cannot be upgraded further.
In short, the Cuirassier although costly and slow for a cavalry unit, is still of use against a variety of targets, but are also somewhat vulnerable to ranged cavalry units and dedicated anti-cavalry units, such as Grenadiers and/or Flying Artillery barrages. The best use thus for your Cuirassiers is to flank enemy infantry and artillery where possible, or to use them in a more defensive manner in protecting your flanks or defending artillery from enemy melee cavalry, particularly from attacks by Hussars or light cavalry.
Towards the 16th century, the increased availability of horses, weapons and armour radically changed how cavalry units were recruited and deployed in Europe. With the gradual disestablishment of the feudal system, the role of heavy cavalry reflected this too, as being no longer the preserve of the highest-ranking nobles: recruitment was also open to commoners and yeomen to make up for a shortfall in manpower, especially after the Black Death of the mid-14th century. Even so, the weapons and accoutrements of such warriors had not completely died out. In many parts of Europe, lance-riding armoured cavalry continued to form the shock cavalry component of many armies and for good reason too: the shock and morale effect of the lance when used mounted in cavalry warfare, combined with its reach, meant that it was a very effective melee weapon against most targets, whether on two feet or four.
All that changed when antipersonnel firearms became more effective weapons for use on the battlefield — the rise of the musket and the wheel-lock meant that firearms could now have greater rates of fire as well as range and projectile velocity, making them a threat even to armoured knights. Thus, heavy knights and demi-lancers gave way to cuirassiers — named so for the breastplate or cuirass which they wore into combat, which was the only piece of body armour that would be retained over in the centuries after the Middle Ages.The heavy cavalry of the period would begin to evolve, first by exchanging lances for swords and pistols, then by stripping away most of their body armour. Helmets and breastplates were still in existence by the 17th century, but by the 18th, even light helmets had gone too.
The cuirass however was retained as it gave these new soldiers an edge in the close combat scraps they were envisioned to fight as well as some limited protection from stray rounds, although it was heavy, cumbersome and uncomfortable to wear, especially during the summer months. The weight of the cuirass was such that even though cavalry tactics had changed, the same people and animals that had classified the mediaeval knight were still being placed in that same role: big and strong men, mounted on big and strong horses. Even so, many French veterans swore by it during the Napoleonic Wars, although by then many cuirassier units of other nations had by then been discarding even the breastplates too. Whether armoured or otherwise, cuirassiers continued to retain their position nonetheless, most notably in the French army, until the emergence of rifled guns, mechanisation, and automatic firearms in the First World War brought an end to the relevance of melee cavalry tactics in Western Europe. Cuirassier-type units still exist in a handful of countries in the Western hemisphere, but retain their 19th-century uniforms solely as parade dress today.