If the empire is modeled after the "Byzantine" or Komnenian era Romans. Cavalry should in fact be much more present as much Like the westerners and Easterners was it's focus.
Problem is, what exactly is the game's period? Is it Late Dark Ages? You can balance the game's combat alot easier if you decide what period are you modeling them after. As generally speaking, Nations that didn't have much heavy cavalry could still counter nations that did simply because they had to. (They're terrain also helped them shape themselves.)
Also I agree with the Battanians. Maybe the Cavalry should be modeled after the Gaels, Javelin throwing Light Horsemen. And the faction itself focused on heavy long-Bows, self-bows (Basically Welsh) and really anything that can impale a man or cleave him in two as the Gaels and Dacians where quite fond of Two handed Swords and Martial prowess of the simple short axes/swords/javelins spear and shield.