The five centuries of early medieval art, from the sixth to the eleventh, was the period of transition in which European art of the Middle Ages was born. Although the period lacks unifying characteristics, four artistic centres are clearly discernible - Irish, Anglo-Saxon, Carolingian and Ottonian. The monastic artist-scholars of these centres left a vast heritage of illuminated manuscripts. But their boundless sylistic explorations, like the Lindisfarne Gospels, are not the only flowering of early medieval art, which encompasses also the work of master craftsmen such as those who designed and cast the magnificent doors at Hildesheim. Although Charlemagne's Chapel Royal at Aachen is one of the few buildings left from this period, the surviving gems of the allied architectural arts of mosaic, carving and fresco give us some idea of the glories of the age.