Entering from the West one first enters an exonarthex, next an esonarthex. Enc. Britt. on narthex: long, narrow, enclosed porch, usually colonnaded or arcaded, crossing the entire width of a church at its entrance. The narthex is usually separated from the nave by columns or a pierced wall, and in Byzantine churches the space is divided into two parts. An exonarthex forms the outer entrance to the building and bounds the esonarthex, which opens onto the nave. Occasionally the exonarthex does not form an integral part of the main body of the church but consists of a single-storied structure set against it. In the early days of Christianity the narthex was the only portion of the church to which catechumens (those preparing for the sacrament of Baptism) and penitents were admitted.
In its center it has the Emperors Portal, which you see here. You get a glimpse from the central dome, and to the left and right of this door are geometric decorations. The church did – except for in the apse- not have mosaics until after the iconoclasm period (843).