Through the Outer Gardens of the Saray - Strolling Through Istanbul: The Classic Guide to the City - page 90

The ground around the church has risen some five metres above
its ancient level and the present entry is through a Turkish porch and
outbuildings towards the western end of the north aisle. Entering the
church, we descend in semi-darkness along a stone ramp to the level
of the interior. At the end of the ramp we pass through the north aisle
and find ourselves at the rear of the church, looking down the length
of the nave. From here we can see that the church is a basilica, but a
basilica of a very unusual type. The wide nave is divided from the side
aisles by the usual columned arcade, but this arcade is interrupted
towards the west by the great piers that support the dome to the
east and the smaller elliptical domical vault to the west. The eastern
dome is supported by four great arches which are expanded into deep
barrel-vaults on all sides except the west. Here we see the transition
from a pure domed basilica to a centralized Greek-cross plan, which
was later to supersede the basilica. The apse, semicircular within, five-
sided without, is covered by a semidome. Below there is a synthronon,
the only one in the city surviving from Byzantine times; this consists
of six tiers of seats around the periphery of the apse facing the site of
the altar, with an ambulatory passage beneath the top row, entered
through framed doorways on either side.
In the semidome of the apse there is an ancient mosaic of a simple
cross in black outline standing on a pedestal of three steps, against a
gold ground with a geometric border. The inscription here is from
Psalm lxv, 4 and 5; that on the bema arch perhaps from Amos ix, 6,
with alterations, but in both cases, parts of the mosaic have fallen
away and the letters were painted in by someone who was indiferent
both to grammar and sense. There is some diference of opinion
concerning the dating of these mosaics, one opinion being that they
are to be ascribed to the reconstruction by Constantine V after the
earthquake of 740, the other holding that they are from Justinian's
time. The decorative mosaics in the narthex, which are not unlike
those in Haghia Sophia, are almost certainly from Justinian's period.
At the western end of the nave, five doors lead from the church
into the narthex and formerly five more led thence into the atrium,
but three have been blocked up. This atrium and the scanty remains
of that at St. John of Studius (see Chapter 16) are the only ones