Pffft, its not really a secret that all-cavalry armies are the simplest way to advance armies in vanilla RTW.

I've done it with Praetorian Cavalry, Gothic Cavalry, Cataphracts, even Round Shield Cavalry. Whether against barbarian warbands or armies of hoplites and pikemen, a single all-out massive charge is generally enough to cause the enemy to break and flee. Even if a few hardier units are more stubborn, local superiority in numbers is easily acheived with half of their comrades running away and they soon break as well...

This is not to say they are 100% effective, but its worked most of the times I've tried it; the AI's penchant for deploying all their forces in a single massive but relatively thin line only aids the success rate. The excessively powerful charges of vanilla cavalry - which would really have been much weaker, because without stirrups charging at full speed meant you were very liable to go flying off your horse - allows them to smash phalanx lines and even assault and garrison cities.

I've tried to avoid exploiting this in my more recent campaigns, but its still rather seductive; a fast-moving army on both maps thats easy to command in battle and has a high success rate.