At the Gate of the Xylokerkus, or the Kerko Porta, the Theodosian Walls come to an abrupt termination, and the line of defence from that point to the Golden Horn is continued by fortifications which, for the most part, did not exist before the seventh century. Along the greater portion of their course these bulwarks consisted of a single wall, without a moat; but at a short distance from the water, where they stand on level ground, they formed a double wall, which was at one time protected by a moat and constituted a citadel at the north-western angle of the city. With the exception of that citadel’s outer wall, erected by Leo the Armenian, the defences from the Kerko Porta to the Golden Horn have usually been ascribed to the Emperor Heraclius.[448] But this opinion is at variance both with history, and with the striking diversity in construction exhibited by the various portions of the works. As a matter of fact, the fortifications extending from the Kerko Porta to the Golden Horn comprise walls that belong to, at least, three periods: the Wall of Heraclius, the Wall of Leo, and the Wall of Manuel Comnenus.[449] Curiously enough, the Wall of Manuel Comnenus, though latest in time, stands first in order of position, for it intervenes between the Theodosian Walls, on the one hand, and the Heraclian and Leonine Walls, on the other. Here, therefore, a question presents itself which must be answered before proceeding to the study of the walls just mentioned. If the various portions of the fortifications between the Kerko Porta and the Golden Horn did not come, respectively, into existence until the seventh, ninth, and eleventh centuries, how was the north-western side of the city defended previous to the erection of those walls? Two answers have been given to this important and very difficult question. Both agree in maintaining that the city was defended on the north-west by the prolongation of the Theodosian Walls; but they differ as regards the precise direction in which the walls were carried down to the Golden Horn. One view is that the Theodosian Walls upon leaving the Kerko Porta turned north-eastwards, to follow the _eastern_ spur of the Sixth Hill,[450] along a line terminating somewhere in the vicinity of Balat Kapoussi.[451] According to this view, the quarter of Blachernæ, which until 627 lay outside the city limits,[452] was the territory situated between the spur just mentioned and the line occupied eventually by the Walls of Comnenus and Heraclius. The second view on the subject is that the two Theodosian Walls were carried northwards along the _western_ spur of the Sixth Hill, and enclosed it on every side. On this supposition, the suburb of Blachernæ, with its celebrated Church of the Theotokos, without the fortifications, was the plain extending from the foot of the western spur of the Sixth Hill to the Golden Horn, the plain occupied now by the quarter of Aivan Serai.[453]