Good post. And not only class, but it boils down to how armies were formed: the king wouldn't pay to equip anyone, he would make a call for his nobles to go to war, his nobles will call their own vassals and so on. Then each noble would come along a small band of knights (or better equiped troops, his retinue) and as many peasants as he seemed fit given the part of the year, his loyalty,...
I remember making a thread awhile back that suggested making this a game feature. You would essentially have a small, permament standing army of professional soldiers and when you went to war, you would issue a call to arms amongst your nobles with the option of varying degrees of pressure, at which point they would gather their own sub-nobles and peasants to war for your cause. After the war was over, they would blend back into the population. In every city you would have a little counter to represent how many men can be called to arms, and their relative calibur. When you did call them to arms, you would have a mish mash of soldiers depending on what has been historically available in your empire--- everything from mounted and armored nobles to sword and leather armed artisans to spear carrying peasants.
So, instead of training every one of your units, you would only explicitly train your professional soldiers--- the rest of your men-at-arms are in the population of your cities, and you could enhance their combat effectiveness and readiness by improving the wealth in your society, providing drill squares and military academies which they can attend, and making weapons and armor more readily available in your society (citizen soldiers would purchase their own weapons and armor.) Have a society where wealth is concentrated in the upper classes? You end up with a massive army of spear armed peasants and a good number of heavily armed nobles. Have a civilization where wealth and access is distributed amongst the people more equitably? You end up with a lot of leather and splint armored soldiers with shields and swords. You could also adopt civics that would allow your citizen soldiers to pillage and scavenge loot from defeated soldiers, which would give them a greater incentive to go to war properly prepared (with perhaps some disadvantages).
Also, the longer your population is off at war, the more of an economic and happiness penalty stacks in your cities, because the economy has been distrubted by the disappearance of workers (these penalties would encourage an "organic" end to wars). The more dire the call to arms, the more additional "less qualified" soldiers are called to the front and the greater the penalties. A dire call to arms would naturally be made in a defensive war, which would also give defenders an innate advantage.
I've always disliked the idea in medieval strategy games of training vast swathes of soldiers who then stand ready for an indefinate period of time. It's a strategy game relic from the days of Civ 1 that most game designers still aren't willing to move past. It's become very stale, and innovation is long overdue on this point.