The Octagon is the foundation of the cathedral of Philippi — an octagonal church built in the 4th century and rebuilt and expanded twice over the following centuries. The plan is unusual and points east-west: an outer octagonal wall, an inner colonnade, and a central altar set under what would have been a high dome.
The site complex around the octagon — including the bishop's residence, a baptistery, and a courtyard with mosaic floors — is among the most complete early Christian episcopal complexes in the eastern Mediterranean. The mosaic in the bishop's reception room, depicting birds and geometric patterns, is still in situ.