Which factor gave constantinople the biggest advantage for trade?

Constantinople is the ancient city and the capital of Byzantine Empire. After the fall of Byzantine Empire, the trade center moved from Constantinople to Venice. It was positioned on the ingress of the black sea and it also used the major silk route in order to trade its products.  

It was the richest city in Byzantine Empire. The fertility of soil which was rich in mineral sources was the main reason for the city to become rich.  

It was agriculture based economy but had inherited military strategies from Rome. Constantinople traded Grains and silk which earned the Byzantine Empire great revenue.

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