ap for cavalry charge?

[Ludens]

On topic, surely one of the main features in regard to damage caused by a cavalry charge is the closeness of the order of the infantry that is charged. ie they will go straight through skirmishers/light infantry and cause far more damage to such a unit, while heavy infantry will act more like a wall.

Agreed that a lance will be literally just as effective against an armoured man than an unarmoured one. However, a cavalry charge against a unit of skirmishers should be a lot more effective than against heavily armoured infantry. If the engine doe not model this correctly, then maybe stats could be modified to assist.

In R:TW the problem was that a single soldier was as able to resist a charge as an entire formation, so open formations were far more resistant against charges than they should be. You couldn't fix this without breaking something else: either cavalry ploughed through both open and closed formations, or it stopped at both open and closed formations. Maybe it works differently in M2:TW, but I don't think so.

Incidentally, is the effect of an AP lance that lightly armoured "barbarian" infantry are more likely to survive a charge than heavier "civilized" troops if total defence value is equal but armour is less?

Yes. AP halves the armour value, so if total defence is equal the soldier whose total defence value is derived less from the armour suffers the least. However if other defence values are equal units with more armour will do better. Extra armour does not make troops more vulnerable to AP weapons, unless it comes at the cost of other defence values.

This is not totally true. Look at Issos where Alexander charged headlong against the new heavy Persian infantry.

But that's such an unusual exception that many historians believe something else took place. The Kardakes were not prepared, Wiki says that Alexander sent the hypaspists in first, and so on. My own theory is that the Kardakes were thrown into disarray by the Agranians appearing on their flank: the difficult terrain and Persian skirmishers were supposed to protect it, but the Agranians had little trouble with either.