In an age when every noble's house was a castle, and when every castle was erected in order to dominate, as well as to defend, the town and the district in which it stood, the Tower of London was erected. The builder of the White Tower was William the Conqueror, who gave the City its Charter but had no intention of giving up his own sovereignty; the architect, as has been already said, was one Gundulph, Bishop of Rochester. Part of the City wall was pulled down to make room for it, and it was intended at once for the King's Palace, the King's Castle, and the King's Prison. It was also the key of London--who held the Tower, held the City. William Rufus built a wall round the Tower so as to separate it entirely from the City and to prevent the danger of a hasty rising of the people: with the same object he gave it a water gate. A hundred years later, while Richard Coeur de Lion was on his Crusade, the moat was constructed. Henry III. and his son Edward I. added to the outer walls and strengthened them. [Illustration: TOWER OF LONDON.] There is a plan of the Tower made from a survey of the year 1597 and published by the Society of Antiquaries. A study of the plan should be made before visiting the place. Remark first of all that the fortress has three entrances only: one at the S.W. angle to the City; one to the river now called Traitors' Gate; and one on the S.E. angle called the Irongate: that it is surrounded by a broad and deep moat which could be filled at every high tide: that from the moat rises a battlemented wall, and that within this first wall is another, flanked with protecting towers; that the City entrance is most jealously guarded by a strong gate first: then by a narrow way passing under a tower: then over a bridge. In all mediaeval castles the first thought was to make it impossible to carry the place by a rush. If we would restore the Tower of Queen Elizabeth to the Tower of Edward III. we must abolish all those buildings which stand on the north and east sides, with those called 'Lieutenants' Lodgings' on the south. The space on the north side of the Keep was the exercising ground: stables there must have been somewhere in this great area; the men at arms would live in the smaller towers. If you will study this plan carefully, you will understand the general arrangement of a mediaeval castle.