These basilica-churches had a center nave with one or more aisles at each side and a rounded apse at one end: on this raised platform sat the bishop and priests, and also the altar. Although it appears that early altars were constructed of wood (as is the case in the Dura-Europos church) altars of this period were built of stone, and began to become more richly designed. Richer materials could now be used for art, such as the mosaics that decorate Santa Maria Maggiore in Rome and the 5th century basilicas of Ravenna, where narrative sequences begin to develop.