Ok, after a careful process of elimination, the French infantry unit "Tarfur" causes the game to crash when he's included in a custom battle, and he crashes it against numerous opponents so it's just him. Not sure if he causes the campaign battles to crash as well but if he did probably nobody ever noticed because nobody plays early French.
I think there are more game crashers on Super Early as well and I'm determined to root them out, then go for the super late units. FYI I haven't found any Early Units that are empty handed that I've tested so far besides the ones I already listed.
As an aside, for your patch, could you kindly lower the upkeep value of Byzantine Cavalry? I think they added an extra digit in there or something, but the upkeep value of that unit is totally out of proportion to its ability in combat
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