Side "B" - Tribal theocracy with dream of world conquest, lags terribly in tech, lacks serious industrial base, thus prefers infantry but can mobilize gigantic, fanatic armies and continue fight even after absorbing huge damage.
Why would they prefer infantry? Even ancient world armies had cavalry and artillery. If it dreams of world conquest, you could make it resemble ancient persia or greece or rome or egypt or babylon in some manner (or all of them). I just beat age of empires 1 (on easy, because I own), so maybe that's why I feel like suggesting this.
For the record
- The only useful infantry I found in AoE in bronze age are heavy infantry (hoplites in-game; which I think were only really greek spartan/athens even though every race in-game uses them), though any empire could devise any sort of heavy infantry with pre-industrial tech if you're feeling creative. The idea behind pre-industry is that literally any variety can come up, since multiple cultures develop and there's less unity because there's less communication and the world feels larger, etc., etc.
In iron age, you can get longswords, but it's very easy to make the transition to phalanx (the best heavy infantry), and cavalry become heavy cavalry and some races can upgrade further to cataphracts (the byzantines used cataphracts, which were heavily armored cavalry that would often charge in and hit and run, I think?). Mounted/Horse archers reign supreme against pretty much everything except buildings, above composite bowmen (longbows don't exist at all throughout most of the world), and artillery consists of ballistae, helepolis (some sorta ballistae that shoots a lot faster in-game), and stone throwers (bronze age catapults) and heavy catapults, which are these massive devices that clog up roads like tanks and were probably hell to maneuver (I could imagine them, besides slinging boulders, running through infantry with gravity behind them in a jam; kinda like what chariots did, when they forged spikes to the axels of their wheels, but were horse powered; horse archers are above all this, simply because they are the most maneuverable and supreme at indirect and direct fighting). Villager siege craft assumes the use of a greater variety of siege tools that supposedly comes around during the iron age (presumably, by game mechanics), since it lets villagers do considerable damage to walls and towers. Catapults are very good at devastating clustered infantry (foot missiles such as bow and slingers and sword and boarders like longswords and broadswords, and then cannon fodder such as short swords and axemen). Ballistae are average, but helepolis are amazing against infantry and can even blow away cavalry that get careless.
Presumably, the anicent world employed ballistae in lines against a visible enemy. With luck, they could hault a cavalry charge (provided it's not big, or too heavily armored). If the bow is highly developed (up to longbow or some sort of state of craft crossbow like a chukonu, ie.), infantry can expect to be using massive shields for cover or digging trenches. Cavalry are only employed in such a situation if terrain is willing (they can effectively flank archers behind their cover). Longbows and the like can make the best armor useless, until you get to rennaissance full plate, where forces can last long enough for better flanks and hope to turn the conflict into a slogging fest (during the 100 years war between england and france, there were reports of melee lasting for days; full plate combined with shield is useful, needless to say).