This martyrium which is an octagonal structure and has a square base is a monument of Saint Philip. 

Saint Philip was one of the twelve apostles of Christ and he was martyred in Hierapolis in 80 AD. Monument was built in the 5th century AD. He and his daughter contributed greatly to the spreading of Christianity in Hierapolis. 

Saint Philip Martyrium is another sign of the influence of Christianity in the city. It was not a church but was used for religious ceremonies.