Battle tactics

Because Medieval 2 Total War has a great battle tactics system, I will create something similar:

Stamina
Heavy types of soldiers will loose if they have to move or fight much faster than the light ones. Especially if commanded to "Run" or especially "Charge" they will loose Stamina really quickly or being long in a melee fight. If stamina is really low, the soldiers will be weaker in melee and not be able to move fast anymore

How does morale drop

 * If attacked in the rear or multiple flanks
 * Attacked by missile, especially artillery
 * If fighting against a unit much better

What can happen if the morale is too low?

 * They can start fleeing. If charged while fleeing, it is guaranteed they will not come back, else they will return once. In a siege they will retreat and fight to the last at the cities last defence, if lower tier unit they will cry for mercy.
 * You can higher their morale by letting the general inspire them.