most powerful faction

[Vincent Butler]

Now it would be a simple modification to enable paved roads for Parthia, right? And they should get a decent melee cav other than Cataphracts.

Slingers probably would do just as well as EI

Defense is too low. According to export_descr_unit, defense for slingers is 4, while EI is ten. And EI are a spear unit, giving them a total +16 attack bonus vs cav, -4 attack penalty against other infantry. Which means that against other infantry, slingers would be more effective, both before and during melee. So they can wear light enemies (including EI) down, before engaging in hand to hand combat if necessary, against infantry, and have more attack than either EI or Hillmen. Slingers actually have better morale than EI as well.

Slingers actually get only two less attack than Hillmen before bonuses/penalties, same amount of men in unit, and Hillmen defense is actually lower than EI. Also spear, so same advantages/disadvantages as EI. So against cav, field EI or Hillmen, EI gain one defense, lose two attack, two lower morale, but extra forty men. If fighting infantry, you might as well be fielding slingers. Their attack is greater than EI vs infantry (who actually unupgraded get no attack vs infantry), and also than Hillmen, who unupgraded get only attack of one vs infantry. So vs Infantry, Slingers get 4 attack, EI get 0 (actually would be -1), and Hillmen get 1. So field Slingers as Parthia to deal with infantry/chariots, but protect them from cav with your infantry, and protect your infantry from their infantry.