But the history of Constantinople cannot be understood unless the extraordinary character of the geographical position of the place is present to the mind. No city owes so much to its site. The vitality and power of Constantinople are rooted in a unique location. Nowhere is the influence of geography upon history more strikingly marked. Here, to a degree that is marvellous, the possibilities of the freest and widest intercourse blend with the possibilities of complete isolation. No city can be more in the world and out of the world. It is the meeting-point of some of the most important highways on the globe, whether by sea or land; the centre around which diverse, vast, and wealthy countries lie within easy reach, inviting intimate commercial relations, and permitting extended political control. Here the peninsula of Asia Minor, stretching like a bridge across the seas that sunder Asia and Europe, narrows the waters between the two great continents to a stream only half a mile across. Hither the Mediterranean ascends, through the avenues of the Ægean and the Marmora, from the regions of the south; while the Euxine and the Azoff spread a pathway to the regions of the north. Here is a harbour within which the largest and richest fleets can find a perfect shelter. But no less remarkable is the facility with which the great world, so near at hand, can be excluded. Access to this point by sea is possible only through the straits of the Hellespont on the one side, and through the straits of the Bosporus on the other—defiles which, when properly guarded, no hostile navy could penetrate. These channels, with the Sea of Marmora between them, formed, moreover, a natural moat which prevented an Asiatic foe from coming within striking distance of the city; while the narrow breadth of the promontory on which the city stands allowed the erection of fortifications, along the west, which could be held against immense armies by a comparatively small force. As Dean Stanley, alluding to the selection of this site for the new capital of the Empire, has observed: “Of all the events of Constantine’s life, this choice is the most convincing and enduring proof of his real genius.” Although it does not fall within the scope of this work to discuss the topography of Byzantium before the time of Constantine, it will not be inappropriate to glance at the circuits of the fortifications which successively brought more and more of this historic promontory within their widening compass, until the stronghold of a small band of colonists from Megara became the most splendid city and the mightiest bulwark of the Roman world. Four such circuits demand notice.