The Roman Empire in the East, though often menaced by barbarian foes, long continued to be the leading European power. Its highest degree of prosperity was reached between the middle of the ninth and the middle of the eleventh century. The provinces in Asia Minor and the Balkan peninsula produced a vast annual revenue, much of which went for defense. It was necessary to maintain a large, well-disciplined army, great fleets and engines of war, and the extensive fortifications of Constantinople and the frontier cities. Confronted by so many dangers, the empire could hope to survive only by making itself a strong military state. COMMERCE AND INDUSTRY The merchant ships of Constantinople, during the earlier part of the Middle Ages, carried on most of the commerce of the Mediterranean and the Black Sea. The products of Byzantine industry, including silks, embroideries, mosaics, enamels, and metal work, were exchanged at that city for the spices, drugs, and precious stones of the East. Byzantine wares also found their way into Italy and France and, by way of the Russian rivers, reached the heart of eastern Europe. Russia, in turn, furnished Constantinople with large quantities of honey, wax, fur, wool, grain, and slaves. A traveler of the twelfth century well described the city as a metropolis "common to all the world, without distinction of country or religion." CHARACTER OF BYZANTINE ART Many of the Roman emperors from Justinian onward were great builders. Byzantine architecture, seen especially in the churches, became a leading form of art. Its most striking feature is the dome, which replaces the flat, wooden roof used in the basilican [15] Churches of Italy. The exterior of a Byzantine church is plain and unimposing, but the interior is adorned on a magnificent scale. The eyes of the worshiper are dazzled by the walls faced with marble slabs of variegated colors, by the columns of polished marble, jasper, and porphyry, and by the brilliant mosaic pictures of gilded glass. The entire impression is one of richness and splendor. Byzantine artists, though mediocre painters and sculptors, excelled in all kinds of decorative work. Their carvings in wood, ivory, and metal, together with their embroideries, enamels, and miniatures, enjoyed a high reputation throughout medieval Europe. INFLUENCE OF BYZANTINE ART