Just started an RTR game with Parthia. Pure horse army is suprisingly devastating, even against the falangitai. It's pure bliss to see the heavy formation start to slooowly, oh so slowly, turn to face a faked charge by the horse archers and then end up being crashed by the tank platoon of classical times; kataphraktoi. Three tons of armored men and equines having buttsechs with the backs of the falanx. All the while other horsearchers pick and isolate other infantry and cavalry units and pepper their backs to shreds. Mop up with kataphraktoi and armoured camel cavalry, Rinse and repeat until the whole freakin army routes. Then it's just a matter of which unit you want to bestow with more experience. Pure cavalry army against a mass of routing units=trail of some very dead corpses.
Rome, here I come!