Granary of the Empire, the area of actual Tunisia and coastal Tripolitania, according to one estimate, produced one million tons of cereals each year, one quarter of which was exported to the Empire. Additional crops included beans, figs, grapes, and other fruits. By the 2nd century, olive oil rivaled cereals as an export item. In addition to the cultivations and the capture and transporting of exotic wild animals from the western mountains, the principal production and exports included the textiles, marble, wine, timber, livestock, pottery such as African Red Slip, and wool. There was even a huge production of mosaics and ceramics, exported mainly to Italy, in the central area of El Djem (where there was the second biggest amphitheater in the Roman Empire). Berber bishop Donatus Magnus was the founder of a Christian group known as the Donatists. During the 5th and 6th centuries (from 430 to 533 AD), the Germanic Vandals invaded and ruled over a kingdom in Northwest Africa that included present-day Tripoli. The region was easily reconquered in 533–534 AD, during the rule of Emperor Justinian I, by the Eastern Romans led by General Belisarius. Middle Ages Sometime between the second half of the 7th century and the early part of the 8th century, Arab Muslim conquest occurred in the region. They founded the first Islamic city in Northwest Africa, Kairouan. It was there in 670 AD that the Mosque of Uqba, or the Great Mosque of Kairouan, was constructed. This mosque is the oldest and most prestigious sanctuary in the Muslim West with the oldest standing minaret in the world; it is also considered a masterpiece of Islamic art and architecture. Tunis was taken in 695, re-taken by the Byzantine Eastern Romans in 697, but lost permanently in 698. The transition from a Latin-speaking Christian Berber society to a Muslim and mostly Arabic-speaking society took over 400 years (the equivalent process in Egypt and the Fertile Crescent took 600 years) and resulted in the final disappearance of Christianity and Latin in the 12th or 13th century. The majority of the population were not Muslim until quite late in the 9th century; a vast majority were during the 10th. Also, some Tunisian Christians emigrated; some richer members of society did so after the conquest in 698 and others were welcomed by Norman rulers to Sicily or Italy in the 11th and 12th centuries – the logical