The Byzantines are primarily a defensive civilization that excels at turtling. However they are very versatile and are perfectly capable of mounting a good offense as well. As a result, they are a perfect civilization for players who are new to the game and veterans who want to try using different strategies reasonably well.
The Byzantines' primary strengths are defense and their ability to effectively counter a variety of troop types (infantry, ranged, and cavalry). Their buildings have bonus hit points which increase with every age advance. This makes knocking down their walls difficult and their defensive structures benefit as well. In addition they have access to all defensive buildings and their counter units have bonuses, making attacking them even more difficult. Technologically they are the one of, if not the, most advanced civilizations in the game, as they can research almost every technology. In the original game they only lacked the Blast Furnace technology, while in the expansion they are missing several others but are still remarkably advanced. They also have a considerable discount when researching the Imperial Age upgrade.
A Byzantine military will likely be composed mainly of mounted units as they have full cavalry technologies (excluding Bloodlines), but due to their large amount of accessible technologies, they can effectively create any kind of army. Their unique unit, the Cataphract, excels at killing infantry but can take on almost any unit due to their large attack and armor. In addition they are virtually immune to bonus damage from Camels and Mamelukes, making these normally potent anti-cavalry units harmless. The Byzantines also possess a strong navy with full naval technologies and have full access to gunpowder units. They also have a faster attacking Fire Ship which helps them defensively in the seas, and makes them slightly better at shore-bombardment in the Castle Age. In addition, the Byzantines' Halberdiers, skirmishers, and camels have a cost reduction. These units give them effective counters to cavalry and archers, working alongside the Cataphracts' devastating anti-infantry ability.
Byzantines are versatile, but have a few weaknesses. They excel defensively but they don't have as many offensive bonuses to aid them while attacking, since they lack Blast Furnace. Due to this, Paladins are one of the best defenses against their Cataphracts, since they are also cavalry and possess high defensive and offensive stats. Their siege weapons are also slightly lacking, missing the Siege Onager and Heavy Scorpion.
Although Byzantines have a complete Stable unit tree, their Stable units are considered weak because they lack Bloodlines and Blast Furnace upgrades. In a one-on-one situation, the Byzantine Heavy Camel Rider loses to a Paladin with the Bloodlines upgrade. The same outcome also applies to the Cataphract. This in turn puts them at a disadvantage against an army consisted of heavy cavalry. Luckily, this is balanced by the fact that Byzantines possess the cheapest Camels and Halberdiers in the game, so Byzantines are still able to defeat hostile heavy cavalry troops by using numerical superiority.
Byzantines have a nearly-complete tech tree, allowing them to do pretty much whatever they want, instead of being limited to one or two particular strategies like other civilizations. Unfortunately, they don't truly excel at any strategy save defense. Typically, Byzantines will try to reach the Castle or Imperial Age as quickly as possible, since that is when their tech advantages shine. They are perfectly capable of Dark Age rushing (drushing) or Feudal Age rushing (flushing) if they want to. They can flush with any Feudal units, although their Skirmishers and Spearmen are cheaper, allowing them to effectively counter an incoming flush. Maybe the best tactic used by Byzantine players is turtling since all their buildings have a large amount of hit points, and them having access to all defensive structures, having the entire Watch Tower line, Bombard Tower and overall strong castles.
In the Castle Age, Cataphracts, Camels and Knights work exceptionally well together. All are fast and mobile, Cataphracts help counter Pikemen and Camels, Camels get rid of opposing Knights who can otherwise give Cataphracts difficulty, and Knights are all-around bulky and good for killing Archers and raiding Town Centers and Castles. Byzantines also get cheaper Camels, making this combination even more effective. With their team bonus increasing Monk heal-speed, the player can also keep some Monks around to make up for the lack of Herbal Medicine.
Castles, Keeps and Bombard Towers make for an effective defense on land maps. On sea maps, Byzantines lack Heated Shot, so they should attempt to win the water with their stronger Fire Ships and complete Dock techs.
Strategy changes in The Forgotten
Their new Tech Greek Fire gives Fire Ships +1 range; they already have good and strong Fire Ships so this new tech gives a bigger advantage at the sea and can take down enemy ships for more distance.
Strategy changes in The African Kingdoms
Byzantines have already a nearly complete tech tree and in this expansion this tech tree is furthered with the addition of the Arson and Arrowslits technologies which in this last case furthers the defensive capabilities of the already strong Byzantine towers. Also, the addition of the Fire Galley in the Feudal Age gives an early edge to the Byzantine navy since they will benefit from the Fire Ship bonus.
Strategy changes in the Definitive Edition
Watch Towers' base HP is reduced to 700 in the Dark Age (in the Castle Age, they still have 1,200 base HP). Despite this, the Byzantines can still perform a Tower rush (and can partially circumvent this nerf, as their HP bonus still applies); it is just a bit harder to make it and succeed. Update 39284 reduced the food cost of the Elite upgrade of the Cataphract and the Logistica technology to 1,200 food and 800 food, respectively, encouraging their use.
Byzantines make great support in team games, as they have several defensive bonuses that can aid their allies. Their HP bonus for buildings greatly helps their team, as it allows them to create strong defensive structures. If an ally attempts a Wonder or Relic victory, is recommended that the Byzantines support that player with Walls, Towers, and Castles (even in the Dark Age, which can also be useful for Gurjara team-mates to safeguard their Mills and the herdables they garrisoned into them).
The Byzantine team bonus makes monk support more effective, as the monks can heal faster. This is especially useful for civilizations that already have good monks or bonuses for monks, like the Aztecs, the Saracens (especially when they perform the Smush), the Slavs, the Spanish, the Teutons (which will make their monks the best healers in the game, as their monks have also a healing range bonus that was fixed in the Forgotten), the Burmese, the Lithuanians (combined with their own bonus to produce Monks quicker) and the Bohemians (they have the Hussite Reforms technology that make their Monks food-only healers, i.e. another handy instrument for "trash wars").
With an open tech tree and a wide variety of tactical options, most team bonuses will benefit the Byzantines at some point of the game. Maybe the best teammates for the Byzantines are the Incas (Byzantine cheaper Halberdiers and Skirmishers will get +2 LOS), Mayans (as their team bonus will make the strong Byzantine walls cheaper), the Vikings, the Japanese , the Malay, the Sicilians, or the Dravidians(if playing on water maps), and the Hindustanis (which will add their anti-building bonus to the cheap Byzantine Camels). It may seem useless on paper, but the Cumans' team bonus (+33% hit points for Palisades) is a great boost in the early-game defense, and it stacks with the Byzantine hit point bonus for all their buildings, making an early turtling tactic much more viable.
Both Berber and Vietnamese allies can deliver quality trash units (Genitours from the former and Imperial Skirmishers from the latter) which allow Byzantine players to "swarm away" their opponents once-and-for-all as those in the Byzantines' case are cheaper. Apart from that, Kasbah from Berber allies makes amassing large Cataphract contingents feasible, particularly in deathmatches.
Compared advantages and disadvantages
Advantages vs other civilizations
Disadvantages vs other civilizations