In about the 5th century the Roman Empire was split into two.
The western empire with its capital in Rome, the eastern empire with its capital in Constantinople (what we now call Istanbul).
It`s "strengths" would have been all those things it inherited from the old Roman empire - a functioning bureaucratic government, trading links which generated wealth, scientific knowledge, military technology and so on.
This goes a long way to explain how it was able to survive for the best part of a millennium in the face of aggressive Islamic expansionism.
