The position of Constantinople made it difficult to attack but easy to defend. To surround the city an enemy would have to be strong upon both land and sea. A hostile army, advancing through Asia Minor, found its further advance arrested by the long, winding channel which the Bosporus, the Sea of Marmora, and the Dardanelles combine to form. A hostile fleet, coming by way of the Mediterranean or the Black Sea, faced grave difficulties in attempting to penetrate the narrow strait into which this waterway contracts at each extremity. On the landward side the line of defense was so short--about four miles in width--that it could be strongly fortified and held by a small force against large numbers. During the Middle Ages the rear of the city was protected by two huge walls, the remains of which are still visible. Constantinople, in fact, was all but impregnable. Though each new century brought a fresh horde of enemies, it resisted siege after siege and long continued to be the capital of what was left of the Roman Empire. [16] MONUMENTS OF CONSTANTINOPLE Constantine had laid out his new city on an imposing scale and adorned it with the choicest treasures of art from Greece, Italy, and the Orient. Fourteen churches, fourteen palaces, eight public baths, and several triumphal arches are assigned to the founder of the city. His most stately building was the Hippodrome, an immense structure devoted to chariot races and all sorts of popular gatherings. There new emperors, after their consecration in Sancta Sophia, were greeted by their subjects; there civic festivals were held; and there the last Roman triumphs were celebrated. Theodosius the Great built the principal gate of Constantinople, the "Golden Gate," as it was called, by which the emperors made their solemn entry into the city. But it was Justinian who, after Constantine, did most to adorn the new capital by the Bosporus. He is said to have erected more than twenty-five churches in Constantinople and its suburbs. Of these, the most beautiful is the world-famed cathedral dedicated by Justinian to "Holy Wisdom." On its completion the emperor declared that he had surpassed Solomon's Temple. Though nearly fourteen hundred years old and now defaced by vandal hands, it remains perhaps the supreme achievement of Christian architecture. [Illustration: SANCTA SOPHIA, CONSTANTINOPLE Built by Justinian and dedicated on Christmas Day, 538 A.D. The main building is roofed over by a great central dome 107 feet in diameter and 179 feet in height. After the Ottoman Turks turned the church into a mosque, a minaret was erected at each of the four exterior angles. The outside of Sancta Sophia is somewhat disappointing, but the interior, with its walls and columns of polished marble granite and porphyry, is magnificent. The crystal balustrades, pulpits, and large metal disks are Turkish.] [Illustration: THE THREE EXISTING MONUMENTS OF THE HIPPODROME, CONSTANTINOPLE.