In the first insula east of the Roman Forum at Philippi. between the eastward extensions of Egnatia and Emporiki streets, is the metropolitan church of Philippi, the famous Octagon, which is dedicated to the Apostle Paul. Secondary cross roads, the parodoi, ensured communication between the two large streets and demarcated the individual buildings that surrounded it.
The Octagon is a characteristic example of building a Christian church on the site of an ancient sanctuary, and a rare example of a complete Early Christian building complex. The first house of worship, a single-naved basilica, was built after the persecutions and shares a common wall with an underground Macedonian tomb and above-ground 2nd century BC heroon of Euephenes, son of Exekestos, an initiate of the Kabeirian mysteries. The founding inscription by the Bishop of Philippi, Porphyrios, on the mosaic floor of the house of prayer, decorated with symbolic scenes of birds, trees, and geometric motifs, informs us that the church was dedicated to the Apostle Paul, and founded in 313-343 AD. At the end of the 4th century, the basilica of Paul was replaced by the octagonal church, which incorporated the earlier mosaic of the basilica into its flooring, decorated with rich marble inlays. This church was preserved with various alterations, until the early 7th century.
The Octagon, which was square on its exterior and octagonal on its interior with the help of its corner conches, had on the east the protruding apse of the sanctuary with its built synthronon and the base of the Altar Table. The interior colonnade atop an eight-sided stylobate supported the galleries and dome. The church had two ambones and a narthex with marble inlaid floor. A stoa surrounded the south side of the square and continued on the east in the form of a ring around the conch. A three-aisled stoa with monumental gate on the Via Egnatia replaced the Roman side street and ensured access to the church's narthex.
West of the narthex of the Octagon and the three-aisled stoa extends a monumental atrium with a fountain building on its west side and rooms that probably belonged to a guest house for pilgrims.