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 Fatimid 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 Catholicism 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 Patriarch of 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 Asian side and very close to Constantinople and Adrianople on the European side very close to Constantinople also which all hold 6 slots.
The Byzantine Empire in 867 in the Old Gods DLC. As opposed to earlier games, this version is substantially easier, allowing to be both more forgiving to start at the Old Gods, or Legacy of Rome standpoints. It is truly easy for any good player, but there will be challenges as in the other start dates. In 867 the Byzantine Empire is one of the most powerful empires, having one of the largest levies in the game. It is also the only Christian empire.
Your main enemy will probably be the Muslims to the South. In this start date you will likely face the empire to the South, the Sunni Caliphate. There is also the Muslims in Italy and Spain (rarely). There is a new enemy that will have to be dealt with through, the Magyars.
The Caliphate starts off with relatively similar stats to you, it is large and powerful. Though you will notice that they have a smaller levy than you, THIS WILL CHANGE. DO NOT declare war on them as soon as your done fighting in Sicily, as they usually can raise a levy equaling your power, (although a little bit less). In addition to that, you may notice, that as one of the great rulers of the Arabian empire, this emperor has claims on every Muslim kingdom, and will call every Sunni leader into a war, if necessary. Another problem is Jihad, usually this will take place if you or other christian-western or even outside forces such as Zorostrians, take charge and start attacking. This is especially prevalent when the Shia rise up, as the Caliphate is responding more to attacks from foreigners.
Jihad. the Byzantine empire has always been a center for attack for the Muslims, especially for Anatolia. which this will likely happen in your game time, here are a few tips to avenging and defeating them.
Naval combat is key. You are one of the few nations at the start of the world with an advanced navy, that can easily hold all of your troops, the Muslims don't have that, but they do have plenty of land, the entire Eastern Mediterranean, that can be attacked. If you constantly breeze along the coast with your ships and troops, you can attack quicker then an Muslims, so divide and conquer, attack small armies, quickly with larger armies. siege, where necessary to hold vital points, that would cause starvation for Muslims is the key for another strategy that is mentioned later on.
Holding the mountains Muslim armies should have an extremely tough time coming at you through the mountains they have high rates of attrition, in doing so, you have a line, a line of defense you can guard with armies and generals, through the mountains, this combined with your Sea tactic, it will create roughly a corner of a square, an angle of defense if you will, that you cannot let a single Muslim through.
Defeating the Caliphate, is relatively easy though. In a standard war, because you have similar levies the best course of action is to usually fight a war of attrition, marking how stupidly dumb the AI is at attrition, make sure you lead the enemy into places it has high attrition, and you don't, such as the mountains, which you own, and are especially hard for armies to travel, meaning they will stay and likely starve as they try to catch up to you. That being said, you will have bonuses if you are defensive in the mountains, and will banish any Muslims you have by putting good generals to use with your professional troops.
My personal favorite time to strike is when the Shia caliphate rises. The Shias will usually rise within a few years after you start form any position in the game, the conditions are almost always met, they provide a huge bases to go to war with, usually they rise in the Tuliniud Kingdom of Egypt, (which is historically correct in future dates). In the middle of war, taking land from both sides almost always gurantee success.
This will be remake the Byzantine empire in its greatness. 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.
The empire is actually one of the more favorable of starting countries. This is especially true if you have the Legacy of Rome DLC. There are normal, Greek-empire to orthodox events (more in Sons of Abraham). For the events of Legacy of Rome, which centers mainly around this country in both start dates, you can go to