Quote Originally Posted by Poor Bloody Infantry
For taking walls, always use heavy infantry. The defenders get a hefty combat boost when fighting on walls so you need to use something considerably heavier than the defending unit and which won't rout easily. Spears are at a disadvantage when fighting other infantry so should be saved for cutting down the cavalry once you breach the walls. I find light infantry to be a liability during assaults as they will take so many casualties from the towers and on the walls and they are susceptible to chain-routs, though they are handy for soaking up the enemy arrows which would otherwise be spent on your expensive heavy infantry
As for your other question, I generally prefer lighter cavalry for flanking due to their greater mobility, although I suppose it depends on how quickly you need them there. I tend to use heavy knights for frontal attacks as then they get their full formed-charge bonus and will crush pretty much anything except heavy spearmen.
Good post. Agree lots with bolded text.