Answer: The fall of the Byzantine empire can be contributed to its military activity. As one of the most powerful empires of that era, the Byzantine had several enemies, which formed alliances and planned several assaults to both its people and its defense lines. Similarly, one of the reasons for the fall of the Roman empire was because of its enemies.
Byzantine culture spread when the empire conquered other civilizations, as well as trade.
Hope this helps,
LaciaMelodii :)