How did the Byzantine Empire survive for nearly 1,000 years, despite the outside threats?

The
Byzantine Empire survived for many reasons. First, it had ample agricultural supplies, and it
had a good infrastructure system that funneled these supplies back to Constantinople. It was
well-defended as wellin the latter days of the empire, Constantinople used some of the heaviest
walls in the medieval world. The Byzantines also used a system of chains to keep their harbors
from attack. The Byzantines were able to utilize Greek and Arabic medical knowledge to help both
their soldiers and civilians. The Byzantine army was fiercely loyal to the emperor and was one
of the best-armed forces of the Middle Ages. The empire's navy also utilized Greek fire in order
to keep maritime invaders at bay.

Though towards the end the Byzantine
Empire was little more than Constantinople, the...