The domed basilica from the 6th century shows elements of the late-antique dome architecture as well as that of Justinian-Byzantine architecture. It has a nave and two aisles, a narthex and an apse. The apse is flanked by a Diaconicon to the south and a prosthesis to the nort (from Wikipedia: “Diaconicon: in the Eastern Orthodox and Greek-Catholic Churches, the name given to a chamber on the south side of the central apse of the church, where the vestments, books, etc, that are used in the Divine Services of the church are kept (the sacred vessels are kept in the Prothesis, which is on the north side of the sanctuary).”)

We have now entered the church and are in the narthex. You can see the light coming through the right and central entrances. From the Wikipedia: "The narthex of a church is the entrance or lobby area, located at the end of the nave, at the far end from the church's main altar. Traditionally the narthex was a part of the church building, but was not considered part of the church proper. It was either an indoor area separated from the nave by a screen or rail, or an external structure such as a porch. The purpose of the narthex was to allow those not eligible for admittance into the general congregation (particularly catechumens and penitents) to hear and partake in the service."