what factors, internal and external, might explain the success of the byzantine empire’s survival for so many centuries? what were the factor that led to its ultimate decline and fall?

The trade network of Constantinople (The capital of the Byzantine Empire), was so big that it could sustained the whole city.

In fact, Byzantium (Constantinople) was the only city which Empire lived on for years. Byzantines lost all of their territory to Ottomans. But the capital protected itself by its huge walls.

Eventually, the invention of huge cannons by Ottomans led to its ultimate decline.