Pro tip, Spartan Pikemen are the best thing you'll ever buy for 510 gold. Just won a quick battle with 13 units of them, god they are awesome. I am having a blast playing this game still, but multiplayer battles start to get boring already due to the extreme supremacy of infantry. Can't do much varied armies when having 75% infantry is basically a must. Apart from the dirt cheap elephants, which are basically a no brainer to get two units of, Spartan Pikemen are probably the most OP thing.
Lack of fatigue is also a problem. Who cares about flanking or devious maneuvers when people can sprint everyone to everywhere in a pinch. Even the phalanxes are extremely mobile and adaptable to the situation. Getting attacked from the back? Sprint them towards the enemy, click at the last second and boom, fully functioning phalanx. The speed of cavalry compared to infantry is also very low and so is their survivability. Well, at least I destroyed a full unit of veteran legionaries once with a single charge from Parthia's Royal Cataphracts.