OK, I remembered how to do the -ian switch and get into those Babylonian armies. My crash in 179 is a completely reliable crash, relating to the battle but happening even before the pre-battle screen comes up, and happening whether I invade Susiana as the Egyptians or the Persians - I must have made it crash 30 times while I experimented with different units. In the end it turns out - very surprisingly - that it is the babylonian heavy missile slingers who are causing the crash: 2 units of slingers, and if they are included (even just one of them) there is always a crash; if they are disbanded before I end turn, never a crash.

There is nothing wrong with the unit per se. I've gone back to the start and fought battles with them fine. My theory is that something about their dismountable nature makes the computer unable to decide where to put them in the reinforcement order and so it crashes when it is trying to present that pre-battle screen including reinforcement order. So it only becomes a problem when - as here - you've got 30 plus units, requiring the surplus to be placed in reinforcement tables in quality order. To test that, I disbanded enough units for the slingers to be in the opening line up with no reinforcements - and bingo, no crash. Ditto no crash when I do an attack out of Susiana with a smallish army including heavy missile slingers.

Are they actually dismountable? The unit-prod says so, but in battle they don't behave as though they are (no "change battle mode" option).

This whole idea of infantry dismounting to be other infantry is new and I'm not sure any mod has tried it for real rather than in theory. If I'm right about the slingers, we may need to look at other infantry:infantry instances of dismounting to see if they have the same issue.

I stand corrected on the Median camels - I never tried to build them, just noticed I kept getting messages that they were available in my African provinces.

I didn't really investigate egyptian temples. I noticed they were worth building because of big morale bonuses, but didn't look into it more than that. I will do so next time. And thanks for the pointers re India-Egypt trade.

Re money, I offer as an idea a really simple change I use in various mods to prevent money getting too plentiful. We all know money gets more and more abundant as the game goes on, so I have a tech penalty built into the build_prod file (call it maintenance or whatever). The castle line is labelled as giving cathedral income, and given minus value income so that as you step up you get an increased penalty (12 steps, so say a sliding scale from -25 through to -300). I think it's better than reducing starting income, because a tech penalty tends to penalise the human player more (inveterate builders) and it deals with the late game riches problem. It's really easy to do - just a set of minus numbers in one column of one row of build_prod.

Interested in your thoughts on my heavy slingers and dismounting theory. It isn't so easy to test definitively because you would need to engineer a big clash of two armies with reinforcements, both before and after changing the unit prod. But regardless of that, it's a plain fact that I can't "dismount" the heavy slingers even though the unit prod suggests I should be able to.