In his absence thirty thousand Avars, who had swept over the Balkan provinces like a devouring flame, broke through the great wall and encamped under the very walls of the city itself. Churches in the suburbs were burnt to the ground and the famous Church of the Theotokos in Blachernae was on the point of being destroyed, when some panic caused the Avar horsemen to retire. The danger was too obvious for the warning to be neglected, and the Senate, which had refused with contumely the offers of the barbarian leaders, allies of the Persian King, drove back the enemy and immediately increased the fortifications by a new wall. This splendid barrier, magnificent to-day in its ruins, stretched from the enclosure outside the palace of Blachernae, at the foot of the sixth hill, to the Golden Horn. It is flanked by three hexagonal towers. A year later, in 627, the Emperor, who dreaded even the sight of the sea, crossed the Bosphorus by a bridge of boats, decked with branches of trees to imitate a forest. Landing north of the city he marched inland and crossed the valley at the head of the Golden Horn--below the "Sweet Waters of Europe"--by a bridge made by Justinian nearly opposite the end of the walls. So along the triumphal way he went, past the new walls that have ever since borne his name, and entered by the Golden Gate, the Emperor who had vanquished the Persians, saved his empire, and brought back the greatest of all relics, the sacred wood of the true Cross, which S. Helena, the mother of Constantine, had found on Calvary. But Heraclius was not to triumph unchecked. The fatal temptation of theological strife conquered even the conqueror of the Persians, and the beginning of the Monothelite controversy dates from the _Ekthesis_ of Sergius the patriarch, a document which, if it were intended to make peace, certainly provoked, war that was not ended, though its area was defined, by the decision of the Fourth General Council, which met at Constantinople in 680, and condemned those who denied that Christ had two wills, human and divine. The dreary years of the latter half of the seventh century may be rapidly summarised. Constantinople saw the settlement of barbarians, Slaves and Balgars, almost at its gates. Emperor succeeded emperor without anyone appearing who was worthy to be the heir of Heraclius. At length in 672 the Saracens, who had long devastated Asia, brought a fleet up the Hellespont and besieged the city. Their total defeat by Constantine IV., whom his people nicknamed Pogonatus (the bearded), was the greatest triumph of the Christian powers against the infidel; it was won, it is said, by the newly-discovered "Greek fire," so long to be the terror of the foes of the Empire. Constantinople proved herself the bulwark of Europe against the infidel. The nations of the West sent their envoys to applaud. Six hundred years later another Constantine was to fall, when his city was at length captured by the followers of Mohammed.