The most important changes to military units are (1) the inclusion of new unit classes, (2) the finer gradations between medieval units, and (3) the progress of pike and shot tactics following the invention of gunpowder. More details are available in the Detailed Unit Guide.
New Unit Classes:
Cavalry are now classified as either Light (move 3) or Heavy (move 2). Light Cavalry have high withdrawal odds and great mobility, but are weaker than other units and should be used for pillaging and killing weak units. They also replace recon units, and should be used as sentries and explorers. Heavy cavalry, typically drawn from professional warrior/aristocratic class, were the dominant military force for much of the Medieval period. Our mod reflects this by increasing the strength of cavalry relative to other units, with an accompanying cost increase. Polearm Infantry were developed primarily to counter Heavy Cavalry. Poor training and equipment hampered them in this role until the development of the "Swiss Pike" at the end of the medieval period. Their relative weakness is offset by their low cost and the ability to conscript these units under a feudal levy. Melee Infantry play an important role, especially in sieges, where they are best used to overcome Polearm Infantry and Archers.
Although technological development is still important, individual new units typically have smaller gradations between them. Thus, for instance, we have four types of archers (adding a heavy crossbow) where BTS has three, and three types of polearms (adding a guisarmier).
Pike and Shot:
Critically missing from Civ 4 is the renaissance battlefield when pikes and early firearms came to dominate the battlefield. We have altered the unit dynamics at this stage to capture the flavor of this period. Here, in brief, is the story of military force we are trying to tell in each era.
Early Middle Ages:
With the rise of feudalism, Infantry are supplanted by Heavy Cavalry.
High Middle Ages:
Crossbows and advanced Polearms threaten the superiority of Heavy Cavalry, but with the development of plate armor, knights and other armored units regain the upper hand.
Late Middle Ages:
Gunpowder starts off weak, useful mainly in sieges; its evolution alongside the longbow and the creation of effective pike formations bring to an end the era of cavalry and ushers in an era of infantry dominance.
The "pike and shot" era sees the gradual rise of gunpowder in all areas of the battlefield. Cavalry gain effective guns and cannon become tactically useful, leading to the classic mixed armies of the Napoleonic era.