Footnote 607: Nicephorus Patriarcha CP., p. 30; where it is named τοῦ Βαρνύσσον: Theophanes Cont., p. 340, τοῦ Βαθύρσου. Footnote 608: Leo Diaconus, p. 129; Cinnamus, p. 75. Footnote 609: Anna Comn., x. p. 47. Nicetas Choniates, p. 719, adds that near the bridge stood a perforated rock, τρυπετὸν λίθον. Footnote 610: De Top. CP., iv. c. 6; see, on the whole subject, Du Cange, _Constantinopolis Christiana_, iv. p. 179. Footnote 611: _Paschal Chron._, p. 720. Footnote 612: Gyllius, _De Bosporo Thracio_, ii. c. 13. Footnote 613: Nicephorus Patriarcha CP., pp. 28-30. Footnote 614: Anna Comn., x. p. 47. Footnote 615: Cinnamus, p. 75. Footnote 616: Chap. 33. Footnote 617: Lib. i. pp. 290, 305; iii. p. 501. CHAPTER XIII. THE SEAWARD WALLS. Owing to the unique maritime position occupied by Constantinople, the defence of the shores of the capital was a matter of secondary importance. So long as the Empire retained the command of the sea, a city accessible by water only through the narrow defiles of the Hellespont and the Bosporus had little reason to apprehend a naval attack. This immunity was, it is true, seriously affected when the Saracens and the Republics of Italy became great sea-powers. Still, even then, the situation of the city rendered an assault with ships an extremely difficult operation. The northern shore of the city could be put beyond the reach of the enemy by a chain extended across the narrow entrance of the Golden Horn; while the currents that swept the Marmora shore were ready to carry a fleet out to sea, or to hurl it against the rocks. According to Ville-Hardouin,[617] it was the dread of those currents that, in 1204, deterred the Venetian fleet, under Dandolo, from attacking the walls beside the Sea of Marmora, after the failure of the attempt upon the fortifications along the Golden Horn. Other natural allies to withstand a naval attack were, moreover, found in the violent storms to which the waters around the city are liable. Such a storm discomfited the great Saracen fleet in the siege of 718.[618] In 825, a tempest compelled Thomas, the rival of Michael II., to withdraw his ships from action;[619] while in 865 a storm destroyed the first Russian flotilla that entered the Bosporus.[620] In the long history of the Byzantine Empire there is only one instance of a successful naval assault upon Constantinople, the gallant capture of the city in 1204 by the Venetians. That victory, however, was due as much to the feeble spirit exhibited by the defenders, notwithstanding the advantages of their position, as to the bravery and skill of the assailants.