To the south lies the Fatimid kingdom of Egypt. They are often aggressive against the Byzantines, but frequent Crusades for Jerusalem can easily distract them and make for an easy war.
Byzantine armies are mostly heavy infantry, as is the case for most Christian factions. In the case of AI versus AI, the Byzantines usually fight off the Seljuk invasion of Armenia, but usually lose the invasion that the Fatimids launch towards either Anatolia or Greece which is launched some time afterwards. From there, the empires fortunes will fluctuate, and a lot depends on the impact of the hordes on the Muslim world and the stability of the Seljuks. But they generally survive to the end game
In the DLC "Legacy of Rome" the Byzantine Empire receives a new type of invasion unique to them: Imperial Reconquest. Using this, the empire could reconquer all of the de jure territories of the late Roman Empire and recreate it in the middle ages. In order to do so, you first need to create the title "Roman Empire " as well as mend the "Great Schism",which will convert Catholocism to a Christian heresy, thus making the process of changing Catholic provinces to Orthodox much easier.
They have their own mercenary unit, The Varangian Guards(don't give them land), which can be used as a first strike unit during the start of a war, giving the player time to gather their levies into a proper army. While upkeep can be troublesome, having a high income county such as Constantinople can help offset the costs. Unlike when playing a Catholic country, the player can appoint their own head of the Orthodox church called a Patriarch, who can't call crusades.
Your capital, Constantinople, is your most important holding as at the beginning of the game it already has four city slots, two castles, and a temple. The Byzantine empire is essentially built around the city. Constantinople is also home to the Patriarchof Constantinople, head of the Orthodox church. Constantinople is a major trading hub between the Black Sea and the Aegean and Mediterranean. The city was originally founded by a petty Greek king named Byzos - hence, the name of the city, Byzantium, which was later applied to the realm of the Emperor that ruled from the city. It fell under Roman rule during their expansion into the East, later becoming the capital of the Eastern half of the Empire during the period of division between senior and junior Caesars. Constantine the Great, the namesake of the city, ruled over it as Emperor in the East. Constantine became the first Christian Roman Emperor. Rome, of course, did not last - several sacks of the city by Germanic tribes ended any remnant of Roman rule over the West. Byzantium, then, became the capital of the Roman empire in the East - and, in fact, the citizens of the city would call themselves "Roman" well into the Ottoman period.
Your secondary cities are Thessloniki on the european side and a trading hub , Nicaea on the aisan side and very close to Constanople and Adrianople on the european side very close to Constanople also which all hold 6 slots.
The Byzantine Empire in 867 in the Old Gods DLC. As oposed to earlier games where the where they would mainly end up fighting in Asia Minor against Muslim foes, in 867 the main problem looming is Bulgaria, which include parts of Wallachia and Hungary, however, since they are Orthodox, they should not be dangerous and are capable of helping the Empire. In the East, Armenia and Georgia can be vassalized to save them the trouble of facing other states. The Muslims in Sicily and the Middle East are not strong as they were in 1070, so they are not big trouble but they can be if the Caliph is allowed to subjugate them (the Caliphate here is an Empire). With good managing skills, one can control the de jure empire, Bulgaria not included, with parts of the Middle East and Sicily by 900 AD.