ed.
             This came to stimulate and challenge architects almost as much as its imagery and design, especially from the early Renaissance onward when they began, through detailed measurement and study to try to decipher its ancient mysteries and make direct use of their findings. Only then did monumental rivals of the Pantheon appear, from the hands of Italian masters.
             Certain points stand out in the discussion of the function of the Pantheon on the history of architecture. The first is the curious but fundamental fact that the Pantheon has never been duplicated or copied, at any scale: each Pantheon-like building is, to a greater or lesser degree, a free interpretation of the concepts established in Hadrian's day. The second important point is that the story of the Pantheon in architectural history divides into several clearly differentiated historical phases, the result of particular cultural and stylistic conditions, each phase characterized by an attitude toward the Pantheon that can be defined quite accurately. A third point is that after twin towers were added to the facade in the early seventeenth century, architects had an additional theme to deal with.
             During the late Roman Empire, Roman architects continued to explore the exciting potential of vaulted buildings, which were common east and west. Rethinking the problem brought some spectacular creative successes, while the design of certain building types, notably mausolea, remained conservative as late as the fifth century. Although temple building declined in late antiquity, there are examples in the Pantheon manner, muted in grandeur but of unmistakable origin.
             In this first or imperial phase of the Pantheon's life in architecture temple and tomb design were most influenced. What was apparently the first copy of the Pantheon was a temple to Asklepios Soter of about 145, built in the old Hellenistic city of Pergamon in Asia Minor. The porch had only four columns; the interm...