With the accession of Tiberius Apsimarus the shore defences entered upon a new era of their history. Admiral of the Imperial fleet in the Ægean when the Saracens marched victoriously from the banks of the Nile to the Atlantic, and alive to the power of the enemy upon the sea, as well as upon land, he was in a position to appreciate the necessity of being ready to repel attack at every point. Hence, upon his return to Constantinople, he ordered the walls of the capital, which had for some time been grossly neglected, to be put into a state of defence.[624] Some eight years later, however, Anastasius II. found it expedient to attend to the seaward walls again,[625] in view of the formidable preparations made by the Saracens for their second attack upon the capital of Eastern Christendom; and so effective was the work done, that, in the great crisis of 718, the city defied a fleet of 1200 vessels. In the spring of 764 an unusual occurrence shook the walls about the point of the Acropolis. The preceding winter had been one of Arctic severity. If the figures of Theophanes may be trusted, the sea along the northern and western shores of the Euxine was frozen to a distance of one hundred miles from land, and to a depth of sixty feet; and upon this foundation of solid ice a mass of snow forty-five feet high accumulated. As soon as the breath of spring liberated the frost-bound waters, a long procession of ice-floes came filing down the Bosporus, on their way to the southern seas. They came in such numbers that they packed in the narrow channel, and formed an ice-pile at the opening into the Sea of Marmora, extending from the Palace of Hiereia (Fener Bagtchessi) to the city, and from Chrysopolis to Galata, and as far as Mamas at the head of the Golden Horn.[626] At length the ice divided again, and as its several parts swayed in the swollen currents, one huge iceberg came dashing against the pier at the point of the Acropolis. Another, larger, followed, and hurled itself against the adjacent wall with a violence which shook the whole neighbourhood. The monstrous mass was broken by the concussion in three fragments, still so large that they overtopped the city bulwarks and invested the apex of the promontory from the Mangana to the Port Bosporus, overawing the city, and crushing, it would appear, the fortifications.