It was one of the wealthiest and most densely populated places in the Late Roman Empire. Anatolia's wealth grew during the 4th and 5th centuries thanks, in part, to the Pilgrim's Road that ran through the peninsula. Literary evidence about the rural landscape stems from the hagiographies of 6th century Nicholas of Sion and 7th century Theodore of Sykeon. Large urban centers included Ephesus, Pergamum, Sardis and Aphrodisias. Scholars continue to debate the cause of urban decline in the 6th and 7th centuries variously attributing it to the Plague of Justinian (541), and the 7th century Persian incursion and Arab conquest of the Levant.