单项选择题

William, Duke of Normandy, conquered England in 1066. One of the first tasks he undertook as king was the building of a fortress in the city of London. Begun in 1066 and completed several years later by William’s son, William Rufus, this structure was called the White Tower.
The Tower of London is not just one building, but an 18-acre complex of buildings. In addition to White Tower there are 19 other towers. The Thames River flows by one side of the complex and a large moat, or shallow ditch, surrounds it. Once filled with water, the moat was drained in 1843 and is now covered with grass.
The Tower of London is the city’s most popular tourist attraction. A great deal of fascinating history has taken place within its walls. The tower has served as a fortress, royal residence, prison, royal mint, public records office, observatory, military barracks, place of execution, and city zoo.
As recently as 1941, the tower was used as a prison for Adolf Hitler’s associate Rudolf Hess. Although it is no longer used as a prison, the tower still houses the crown jewels and a great deal of English history.

When discussing the Tower of London the author’s tone could best be described as （）.

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