First Byzantines, in term of generic design they should get a mercantile bonus (the crossroads of all trading routes), stylish walls in Age 1, bonus on siege units (not the gunpowder ones) and infantry.
They shouldn’t have a focus on cavalry, cataphract having a gimmick / tactical ability instead. Byzantines always avoided direct confrontation with western european heavy cavalry for exemple. Similarly to muslim and turkic empires, they used more tactics or harassment.
Byzantines shouldn’t have too much naval focus. Most of their Empire logistic in the covered period was about defending their lands. Ship fires were great, and shined in some battles, but when the Byzantine-Venetians alliance was ruined in a naval battle by Normans, they were never able to recover a strong navy, because not their priority. So again → fire ship, a good gimmick unit, but it don’t make them a water civ. Maybe some religious focus.
Byzantines / Eastern Roman Empire, from 6th century in term of representation (they built the first wonder of medieval age, hagia sofia) for Age I, when Byzantines were more Romans than Greeks. Time of Justinian, their clashes with early islamic empires, the visigoth, andalusian… The famous Theodosian walls which were already built in 5th century, but continued to be elaborated. The huge chain which blocked and protected the harbour … An amazing defensive system for the period. So a strong defensive focus for Age 1, being about the Romans technical heritage. Hagia Sofia as Landmark to fit with accurate timing. As wonder it would appear really too late…
To 8th-9th century in term of representation for Age II. Time of Constantine VI & Irene, the clashes of byzantines with great muslim empires, with the norse, the franks. They could get their cataphract units here.
To 10th-12th century in term of vibes for Age III. Byzantines were the trigger of the first crusade (the call for help ), clashes with seljuks turks (battle of manzikert), Basil II and the Macedon Dynasty and all the related violent clashes against Rus and Bulgarians, and their conversion to orthodoxy.
Byzantines being more and more hellenized because most of their population being greeks.
Wars with Normans. Age 3, being more about byzantine crusades and conversion and their numerous wars, they should get their fire throwers on ships.
13-14th for Age 4. Paleologos period. The decline of a symbol of the period. Fourth crusade, Venetians are no more their allies and betrayed them, crusaders make Constantinople falls in fourth crusade. Most of byzantine moved from constantinople into other places, like in greece mainland. Their numerous clashes with severals turkic and muslim poeple. Byzantine are always invested in a lot of events of the period, even if weakend, they have an important political role, rely a lot on mercenaries and their vassals , and rely a lot on their mercantile importance. → so weak focus on gunpowder (they missed this devellopment because bankrupted a bit before their fall to Ottomans)So good money-mercantile focus in term of gameplay. They could get their varangian guards here, in reference of their last shot of glory with last emperor.
Until 15th when they are totally conquered by Ottomans
Second civ would be the Turks, from early turks which were nomads, with cavalry and mounted archers. To the persianized turks, their ghulams (slave-soldiers), and the devellopment of important islamic and scientific infrastructures, to the Seljuks Empires, eventually already unlock Sipahi unit with them. And finally in Age 4, Ottomans, with a focus on gunpowder : janissaries and bombards