 Complete the following phrase:  During the First Bulgarian Empire the fortress was part of a system of fortifications providing for the defense of Pliska and Preslav, capital cities, and the religious centre of Madara. It then functioned as a minor fort during the 10th12th centuries, as compared to the glory, economic prosperity and military might it had during the 4th6th centuries. In the 13th century it again prospered as a political and economic entity of the reborn Bulgarian Empire. When the Byzantines temporarily took control of Preslav in 1278 during the 