Around the Blue Mosque - Strolling Through Istanbul: The Classic Guide to the City - page 125

Marmara defences consisted of a single line of walls 12-15 metres
high studded with 188 towers at regular intervals. These walls
stretched from the Marble Tower to Saray Point, a total distance of
eight kilometres, and were pierced by 13 sea-gates. At Saray Point,
the Marmara walls joined up with those along the Golden Horn,
thus completing the maritime defence system. Although much of
the fortifications along the Marmara have been destroyed in recent
years, that which remains is still impressive, particularly the walls and
towers along our present itinerary.
Almost immediately in front of SS. Sergius and Bacchus is a small
postern gate doubtless for the use of the monastery that was attached
to the church. Upon closer inspection, we find that the posts of the
gateway are carved with a long inscription in Greek, containing
a conflation or cento from Habakkuk and Psalms. It seems to be
generally agreed that these inscribed doorposts once formed the base
of the celebrated equestrian statue of Justinian which anciently stood
in the Augustaeum.
A short distance beyond this gate we come to the ruins of
another and grander postern, whose Turkish name is Çatladı Kapı,
or the Cracked Gate. The marble sides and archway of the gate are
finely carved with acanthus-leaf decorations as well as with a large
monogram of Justinian. This postern is probably the one which was
called the Imperial Marine Gate, since it appears to have been one of
the entrances from the port of Bucoleon, the private harbour of the
Great Palace. It was also called the Porta Leonis, from the statues of
the two lions which stood on the façade of the Palace of Bucoleon,
one of the seaside buildings of the Great Palace. (These are the lions
which we saw in the Archaeological Museum.) The main entryway
from the port to the palace was by a monumental staircase in the
huge tower just beyond the Çatladı Kapı. As we pass this tower we see
all that now remains of Bucoleon: the eastern loggia of the palace with
its three marble-framed windows and a vaulted room behind them.
Below the windows some projecting corbels indicate that a balcony
ran along the façade, suspended over a marble quay below. Notice the
curious-looking row of large square marble slabs built into the lower
part of the wall; if you insert your hand under them you will find that