Again: the tower S and the tower N block the windows in four of the compartments C. Surely the same builder would not thus go back upon his work. Once more; the loopholes in the stairway-turret afford no light in their present position, the lower pair being closed, the upper pair forming entrances to the =L=-shaped chamber. This is not an original arrangement. In view of such peculiarities, the following conclusions regarding these buildings seem the most reasonable, in the present state of our knowledge: (1) The wall AA was at one time the only erection here; and the two galleries, constructed in the thickness of the wall formed with their loopholes two tiers of batteries, so to speak, for the discharge of missiles upon an enemy attacking this quarter of the city. A similar system of defence was employed for the protection of the smaller residence forming part of the Palace of the Porphyrogenitus,[523] and for the protection of the Palace of the Bucoleon, situated on the city walls near Tchatlady Kapou.[524] When precisely the wall AA was erected cannot be determined; but, judging from its height, and the manner in which it was equipped for defence, the probable opinion is that this was done after the Palace of Blachernæ had assumed considerable importance. Possibly, the work belongs to the reign of Anastasius I.[525] (2) At some later period the wall BB, equipped with buttresses within and without, was erected to support the wall AA. The demand for such support was doubtless occasioned by additions to the Palace of Blachernæ, which already in the tenth century comprised several edifices on the hill behind the wall AA.[526] As BB superseded the original function of the galleries in AA, it was a matter of little moment how many of the loopholes in the latter were more or less masked by the buttresses built transversely between the two walls. It would be enough to retain a few loopholes to light the galleries. At the same time, advantage was taken of the buttresses to construct, in the space between AA and BB, three stories of chambers, for such purpose as the authorities of the palace might decide. (3) The manner in which the towers S and N block the windows in four of the compartments C is evidence that these towers were additions made later than the age of BB. This view is corroborated by the marked difference between the masonry of the towers and the masonry of the wall BB, against which they are built. (4) The towers S and N are so different in their respective styles of construction that they cannot be contemporaneous buildings.