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The main reason why the Byzantine Empire lasted for nearly 1000 years after the fall of the Roman Empire is because it was simply impossible to breach the walls of Constantinople until the advent of gunpowder artillery. Constantinople had 3 layers of defensive walls and a large moat. Constantinople sat in the middle of a trade route, sea, and land. Its wealth came from trade and its strong military. Constantinople remained secure and prosperous while cities in western Roman empire crumbles.

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