To visitors walking uphill within Tsarevets Fortress to Ascension Cathedral, the structure looks like any medieval church, following a Byzantine design and topped with crosses. But the interior is unlike that of any other Christian building. In the fifth century, the hilltop was already the site of a Roman basilica, and the cathedral was built in the 11th and 12th centuries, during the time of the Second Bulgarian Empire, among other buildings that made the stronghold a center of political and religious activity. But by 1393, troops of the Ottoman sultan conquered…