Constantinople was a prime hub in a trading network that extended across Europe, Asia and North Africa. The most important foundation of the Byzantine economy was trade. Grain and silk were two most important commodities. Goods and products were brought in Constantinople from Russia, China, India, Venice, Genoa and many other places. Also Byzantine coin solidus was the standard currency for the Mediterranean basin, which was important for medieval economy.
All this changed with the 4th Crusade ( in 1204 ), which was an economic catastrophe for Constantinople.