The spot chosen was the angle at the eastern corner, just where the wall turns sharply inland from the River, and no position round London could have been better chosen. In the first place it guarded London from the river approach, ready to hold off any enemy venturesome enough to sail up the Thames to attack the city. But also, and this undoubtedly was what was in the mind of the Conqueror, it frowned down on the city. A formidable Norman Keep was erected, with walls 15 feet thick, so strongly built that they stand to-day practically as they stood 900 years ago, save that stone-faced windows were put in a couple of centuries ago to take the place of the narrow slits or loopholes which served for light and ventilation in a fortress of this sort. [Illustration: GROUND PLAN OF THE TOWER.] To understand the Tower of London properly (and we really want some idea of it before any visit, otherwise it is merely a confusion of towers and open spaces without any meaning) we must realize that it consists of three separate lines of defences, all erected at different times. The innermost, the Keep or White Tower, we have touched upon. Beyond that, and separated from it by an open space known as the Inner Ward, is the first wall, with its twelve towers, among them the Beauchamp Tower, the Bell Tower, the Bloody Tower, and the Wakefield Tower. Then, beyond that again, and separated by another open space known as the Outer Ward, is yet another wall; and still beyond is the Moat, outside everything. So that any attacking army, having successfully negotiated the Moat, would find itself with the outer wall to scale and break, and within that another inner wall, 46 feet high. The garrison, driven back from these two, could even then retire to the innermost keep, with its walls 15 feet thick, and there hold out for a great length of time against the fiercest attacks. So that, you will readily see, the Tower was a fortress of tremendous strength in days before the use of heavy artillery. The outer defences were added to William’s White Tower from time to time by various monarchs. The first or inner wall, 8 feet thick, begun in the Conqueror’s days, was added to and strengthened by Stephen, Henry II., and John. The outer wall and the Moat were completed by Henry II.; and the Tower thus took its present shape. Most of our Sovereigns, from the Conqueror’s time right down to the Restoration, used the Tower of London. Kings and Queens who were powerful used it as a prison for their enemies; those who were weak and feared the people used it as a fortress for themselves. This latter use of the Tower was particularly instanced in the reign of Stephen—an illuminating chapter in the story of London.