This ancient road connected Prusa ad Olympum (a city celebrated for its warm baths) in Bithynia to Cyzicus. This road diverted from D4 at the border and moved further west. Once in Bithynia, the road merged with C13 before arriving in Prusa ad Olympum. David French labels this road C1 on his map of Asian roads and A1 on his map of Pontus and Bithynia.
Cyzicus was an unfortified city in existence already in the Hellenistic period. Home to three antique harbors, Cyzicus was a center of commerce. It is on the site of modern Erdek (40.38, 27.89).
Prusa ad Olympum was on the site of modern Bursa (40.1828985, 29.0638185).