Byzantium and the age of gunpowder
Post Gunpowder
After the development of gunpowder, Byzantium has a few new options, though not many. The campaign selection screen says they are lacking in late gunpowder units, and it's not lying. The only land gunpowder units the Byzantines can build, as far as I can tell, are bombards. They get no better cannons that these, and those aren't that great. They are very slow firing. As far as I can tell, Byzantium gets no handgunners/arquebusiers/musketmen at all, nor any firearm equipped cavalry.
On the naval standpoint, the development of gunpowder enables the building of the dockyards, which in turn enables the building of the Laternas, a cannon-equipped war galley. It has a crew of 75 (on large unit sizes) and an attack of 18. However, this ship type is buildable by other factions as well. I've seen Laternas in the hands of the European Mediterranean factions like the Papal States and the Milanese, so now naval parity has been achieved. Byzantium no longer has a naval advantage in either crew size or shipboard weaponry.