In regards to Constantinople and the Byzantium time period, what was so important about it?

After the collapse of the Western Roman Empire, the
Byzantine Empire was the sole remaining "Roman" Empire. Much of Roman tradition that was
preserved had been preserved at Constantinople, the capital of Byzantium. Byzantine
scholars had preserved a great deal of both Roman and Greek knowledge that was lost to
the West. It also occupied a unique geographical location on the Golden Horn overlooking
the Straits of Bosporus which made it easily defended. It was surrounded on three sides
by water and on the land side by a wall so thick that a team of horses could be turned
around on its surface. Constantinople also uniquely was situated where Eastern and
Western merchants and caravans congregated. As a result, it became extremely wealthy as
a trading city. It was the extreme wealth of Constantinople that tempted the Crusaders
of the Fourh Crusade to sack it in 1204.  It's importance as a trading city; is
emphasized by the fact that only after it fell to the Turks in 1453 were serious
attempts by Western Europeans made to circumvent that city and trade directly with the
West.

Constantinople was so well known that its residents
simply referred to it as Istimbolin, Greek for "in the city."  This
name was corrupted by the Turks to give the city its present name:
Istanbul.