Criterion (iv): The Christian monuments of Hierapolis, erected between the 4th and the 6th centuries, constitute an outstanding example of an Early Christian architectural group with a cathedral, baptistery and churches. The most important monument, situated outside the north-west wall of the city, is the Martyrium of St. Philip. At the top of a monumental stairway, the octagonal layout of the building is remarkable because of its ingenious spatial organization. Radiating from the central octagon are chapels, polygonal halls and triangular rooms, which combine to culminate in a square structure encircled by rectangular cells bordered with porticoes.