"Agincourt would have been completely different if the french had taken the "defeated english peseants" seriously. But then that could be said about most battles.[/QUOTE]"
This is one of five misconceptions that are common about medieval armies.
First, let me start by pointing out that how vast the medieval era was in time and space. Therefore, recognize that you have vast lattitude for your campaign. Modelling forces on organized mongol cavalry armies, ill organized turkish raiders, viking boat raiders, rus river trader/raiders, byzantine light cavalry, norman knights, french knights, english yeoman and longbowmen, a central european feudal levy, Vlach (rumanian) light cavalry, Genoese mercenary light crossbowmen, highly motivated and organized teutonic knights and knights templar, or high spirited italian mercenary heavy horsemen from the city states and Swiss pikemen are all acceptable.
Second, if there is a central point to remember it is that the rulers were constantly cash-strapped and (quality) manpower short to varying degrees because they lived in such a structurally backward era without developed notions of taxation, economics, patriotism etc. Individual orders of knighthood, say the templars or the teutonic knights in the baltics, could develop a coherent ideology to motivate their knights, sustain the funding needed to support knights, the accompanying men at arms and footmen, skirmishers, etc., and sustain an army in the field. Other rulers would constantly run out of money, have to disband troops (or have them leave in disgust unpaid, pillaging as they go), declare bankruptcy etc. Normal rulers also often had relatively unmotivated armies (unlike greek hoplite armies of citizen soldiers) who wanted to perform their feudal service, plunder and leave. Therefore, the "average" medieval army was relatively disorganized and "ad hoc" compared to even colonial american armies and way disorganized compared to a modern western army or a roman republican army. In addition, because of this cash-strapped and ad hoc nature, medievael armies had great difficulty taking well-defended serious fortifications. This is why the castle was (almost) king until the development of gunpowder. This is also partly why regional nobles could defy their king as they knew that the king would be hard-pressed to take their castle before the development of cannon.
Third, from time to time, a strong ruler with a decent treasury and popular support of his people COULD from create a medium-sized decent army with a feudal core, a strong yeoman-like infantry, and large numbers of effective mercenaries. For example, both William the Conqueror's army and his opponents (the Saxons) at Hastings were fairly large (maybe 8,000 men) and reasonably effective though they were quite different in composition.
Fourth, effective (trained motivated free men) infantry makes a core to an army that is VERY difficult to break and is generally much more effective than a chivalric group of knights and a peasant rabble levy of infantry. However, large numbers of trained motivated free men were not totally common in all societies. For example, the English archers and men at arms did NOT get lucky at Agincourt, they also annhilated the French at Crecy and Poitiers and never lost a major battle until the french began using cannons. Flemish bourgeous infantry without archers crushed the French chivalry about the same time in similar fashion. Skilled swiss pikeman humiliated the cavalry, mercenaries and levies from austria. Needless to say, motivated Roman legions and Greek Hoplites would have easily done the same thing in a set piece battle against French knights. Remember, motivation is the heart of an army. If you are searching for a modern analogue, think of it as the difference between Israel (highly trained, motivated free men fighting to avoid genocide) and the Arabs (unfree men living in backward despotic regimes, fighting for the glory of their tyrants).
Fifth, what knight-centric armies were truly good at was a combination of skirmishing, raiding, pillaging and serious castle building to secure a territory. For example, the Norman knights who were far better warriors than the traditional French chivalry conquered Southern Italy and Sicily and were the heart of the Crusader armies that held a good chunk of the Holy Land while outnumbered by 10:1+. Intelligent and less scrupulous knights like the Normans knew that the weakness of feudal infantry was its relative immobility and its inability to take fortified places. At Hastings, the Normans did not crush the elite Huscarle heavy infantry of the Saxons, rather, after a long battle, one of their archers managed to shoot the Saxon King in the eye. In short they were opportunists and did not embrace romantic chivalry.