The thing worth noting with chariots is that there are two sets of defence ratings: one for the driver/passengers, and one for the chariot itself. The Scythed Chariot for example has no armour for the chariot, but 18 (same as a cataphract) for the driver. You don't see this rating, but I assumed this rating was used against missiles, whereas the chariot rating was used for close quarters/melee fighting. That would explain why scythed chariots can be difficult to kill when firing missiles, but much easier to kill when charged into a basic unit of troops.
However, before posting that, I thought I'd better do some tests to make sure. And it actually seems the stats for the drivers defence are indeed ignored completely. Setting the hitpoints to 1, I copied the scythed chariots, so I had 2 variations to test. The first had the normal stats (18 armour) but 1 hitpoint, the second had only 1 armour for the driver and 1 hitpoint. By using just 1 hitpoint, I could therefore test if a single volley of arrows would prove whether or not driver armour matters. If it did, the unit with 18 armour would die slowly, the other would die very fast. Lo and behold, both units suffered massive casualties in each volley. Both units also fell apart when they crashed into the Town militia stood in front of the archers.
I then changed the 2 sets of scythed chariots so the chariot itself had 18 armour and the driver 1. This time the chariot withstood several volleys of missiles, plus withstood much of the attacks from the town militia. I then went back to adding massive amounts of armour to the driver and reducing chariot armour to 1, then just set my town militia on the chariots to see if in melee, driver abilties did matter at all - and it was a resounding no.
With this in mind, it seems better to give the chariots heavy armour but just 1 or 2 hitpoints. The game seems to strongly rate units with high hitpoints, thus ruining the results in auto-calced battles. I don't know if it's common knowledge, but it's new to me.
Sorry I've kind of gone into modders discussion mode which doesn't help the original poster whatsoever. It does, however, make me want to play with chariot stats some more. So excuse me, I've got some scythed chariots to steer over the heads of some unruly peasants.
(PS: I haven't tried them, but Armenia would be ok for a new RTW challenge. There's quite a bit of variety to their roster once you get going. And although their infantry isn't all that great, it's sometimes fun just to spam rubbish armies. The Seleucid campaign is great, but similar in some ways to the Carthage one you completed: ie, very tricky start and lots of battles, but then incredibly powerful elephants to finish everyone off)
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