I here group pictures that orignally were in a series describing the terrain as I walked through it. I do not have the time to overwrite captions, so they may be awkward.
The Martyrion was the most important Christian cult building at Hierapolis. The structure is composed of a central octagonal room. Eight rectangular rooms open onto it, each with three arches supported by columns on marble octagonal plinths. The number eight, which lies at the base of the entire complex architectural geometry, has a strong symbolic significance. The arches bear Christian symbols with circles (cross with a star, Chrismon [Wikipedia: A chrismon is one of number of Christian symbols intended to represent aspects of the Person, life or ministry of Jesus Christ and the life, ministry or history of the Christian Church through a single image, emblem or monogram. The term "chrismon" comes from the Latin phrase "Christi Monogramma",]). The central room, once capped by a wooden dome, contains a synthronon (the stepped semicircular benches for the clergy and the bishop during the liturgy). The complex church plan was included in a square, which is surrounded by 32 rooms aligned along the sides. The building must have held the earthly remains of the Apostle Philip. The martyrion was built in the early 5th century AD, perhaps by an architect from the court of Constantinople.