The main entrance of the White Tower opened out on the first floor of the Keep, whence a turnpike staircase led up to the second floor, and downwards to the basement with its dungeons. The mural corridors or passages in the thickness of the walls which encircle the State rooms, are so narrow that only one person could pass along them at a time, which would have been of great advantage in case of an attack on the building, for a small number of men could have defended the White Tower against a host of besiegers. The Normans showed a rare skill in the strategic construction of their strongholds. For instance, in the ruined Castle of Arques near Dieppe, a contemporary building, the plan of its Keep resembles in structure that of the White Tower. These Normans were master builders, and the skilful manner in which they concealed the entrances to their fortresses is well worth study. Their keeps were generally rectangular, and in no instance is the entrance of these towers on the ground floor, or in a conspicuous part of the building. At the Castle of Arques the entrance to the Keep is carefully concealed, as was the case with the White Tower, and is fully 30 feet above the level of the ground, besides being hidden and protected by a massive and lofty wall which forms a part of the Keep. A tortuous passage leads into the heart of the building, but before it could be entered, a very long and almost perpendicular staircase had to be mounted. This staircase commenced in the thickness of the wall of one of the outer counter-forts, placed at the northern angle of the fortress, which wound along the inner face of the Keep, giving access to a landing, beyond which was the passage that led into the fortress. Before the kernel of the Keep could be reached, another narrow passage, cut out of the thickness of the wall, had to be passed; this passage was on the level of the first floor. This style of defensive construction was introduced by the Conqueror and his clerical architect, the quondam monk of the Abbey of Bec in Normandy, who ended his life as Bishop of Rochester; and to these two men we owe the solidity and time-defying strength of the great Norman White Tower.