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

There are several reasons why the Byzantine Empire
survived for nearly 1000 years despite outside threats.  First, the Byzantine emperors
were very strong and had absolute power. The support of the Byzantine Church reinforced
the strength of the emperor.  The government and military of the Byzantine Empire were
backed up by an effective group of administrators. Besides having a strong effective
government, the Byzantine army and navy were also very skilled.  The Byzantine Empire
was divided into military districts ruled by a general who answered to the emperor. 
Soldiers in the district armies received land as payment.  Fighting for the land on
which they lived gave them added incentive to be loyal and brave.  The military leaders
were very capable and led highly trained troops.  Military techniques were advanced. 
They used mirrors to flash signals for attack and retreat, and a medical corps was
established to care for the wounded.  The Byzantine navy also was very powerful. In one
campaign against Crete, 1330 ships were launched.  The navy used “greek fire”, a
combination of sulfur and naphtha that could burn ships on the water.  Finally, the
Byzantine Empire had a stable and thriving economy, with Constantinople becoming a
center of trade and manufacturing.  This brought wealth to the empire which helped pay
for the expense of defending the empire against its enemies.