Historian mode.
In all seriousness, I do learn a lot about the middle ages, especially the High-Late (1067-1300, I'd say) periods. What interests me the most is pretty much what all fun games have nowadays: Warfare. I am glad this thread was posted so I could say all of this.
The often misunderstood aspect of this 'warfare' is the recruitment... nah, it ain't like in fantasy where armies are paid volunteers (I'm looking at you, Elder Scrolls) and that is how the whole thing goes... it's a little more complex! Here, kids:
Levies: A common and technically free way for the monarchy to bulk up his army! You can just go grab some peasants and force them into your army with the excuse of 'feudal obligation'. Many peasants, especially the upper class/married ones, could indeed arm themselves quite well and if they didn't want to fight, too bad! The average peasant soldier of, say, the High Middle Ages, would be a relatively disciplined soldier with perhaps a longbow, helmet, gambeson and a sidearm. Wealth was diverse for the commoners and nobility, they were not all the same (that's right, peasants could make a fine living if they had the minimum money and status). These were as close to 'average soldiers' as you could get with the exception of...
Mercenaries: Ah, yes, professional soldiers who equip themselves! Imagine that! They would fight for you and were most popular in the late middle ages and peasant levies were rarely in demand during the whole Hundred Years War. England was a great supplier of mercenaries and many of its soldiers were. England did have many victories and earned a lot despite losing the Hundred Years War, therefore the mercenaries were more loyal to them. These types of soldiers are often men-at-arms, and sometimes knights would mix with them, though wouldn't be the 'important guy of the crowd' as knights were not the best-best during that time.
Knights!: Yep, the elites! They could volunteer or be told to support the armies. Knights were easy to recruit during both war and peace times. You could instantly knight a man in your army if you needed more knights, which was good for both you and them, or train a child at the age of about seven until he's an adult and ready to fight! There were other exceptions, such as Joan of Arc, but we're talking true knights that would be trained since childhood, hoorah!
And that is how recruitment works. Now, remember that wartime was complex, so now I am going to stop before this gets too big.