The Byzantine economy was among the most robust economies in the Mediterranean for many centuries. Constantinople was a prime hub in a trading network that at various times extended across nearly all of Eurasia and North Africa. Some scholars argue that, up until the arrival of the Arabs in the 7th century, the Eastern Roman Empire had the most powerful economy in the world. The Arab conquests, however, would represent a substantial reversal of fortunes contributing to a period of decline and stagnation. Constantine V's reforms (c. 765) marked the beginning of a revival that continued until 1204. From the 10th century until the end of the 12th, the Byzantine Empire projected an image of luxury, and the travelers were impressed by the wealth accumulated in the capital. All this changed with the arrival of the Fourth Crusade, which was an economic catastrophe. The Palaiologoi tried to revive the economy, but the late Byzantine state would not gain full control of either the foreign or domestic economic forces.One of the economic foundations of the empire was trade. The state strictly controlled both the internal and the international trade, and retained the monopoly of issuing coinage. Constantinople remained the single most important commercial centre of Europe for much of the Medieval era, which it held until the Republic of Venice slowly began to overtake Byzantine merchants in trade; first through tax exemption under the Komnenoi, then under the Latin Empire.