In the sixth century, during the reign of Justinian, the Byzantine Empire had its maximum extension and as such Constantinople as the center of the empire flourished, reaching its apogee.
Over the centuries the Byzantine Empire declined, first after losing the Battle of Manziquerta, against the Seljuk Turks, which resulted in the loss of Anatolia, a very fertile region of present-day Turkey, and later when Constantinople was sacked by the Fourth Crusade.
The city, which had 1 000 000 people, had only 40 000 inhabitants by the end of the first millennium and ended up being completely surrounded by the Ottoman Empire during the centuries that followed.
Only its excellent, almost insurmountable walls made the city stand on Byzantine control until the middle of the fifteenth century.