Tower of London
This is the White Tower. It was the first building on the site of the Tower of London. William the Conqueror had it built in the 11th Century.
For me, no visit to London is complete without a visit to the Tower of London. It represents 1000 years of English history. It was a royal palace for several hundred years. It was a royal dungeon. It was the site of many beheadings. It is still home to the royal jewels, which are on display.
The tour by one of the yeoman warders is a must. These former soldiers live at the tower with their families. They know the history with all its gruesome details, and share those details with a sense of gory humor.
If you have one day in London, spend it at the Tower.