One of the premier tourist sites in London, the Tower of London has been a palace and a prison. At other times it also housed the Royal Mint, a royal menagerie, royal armouries (since moved to Leeds) and royal observatory.
The White Tower is the focal point and was built by William I to protect London and impress his subjects. Later royals kept adding to the site until it enclosed 18 acres.
One of the more gruesome aspects of this site is that is was used for executions, some notable ones being Lady Jane Grey, Ann Boleyn, Sir Thomas More and the Earl of Essex.
The Beauchamp Tower is interesting to visit as you can see inscriptions on the walls placed there by prisoners awaiting their time with the executioner.
The Crown Jewels are an attraction which many people line up for ages to see, I've yet to do so and probably never will as I'm just not interested. The lineups can stretch for a very long way.
Be sure to see the Ravens who are a symbol of the British Empire. It's said that if they leave the Tower of London, the British Empire will crumble.
If you wish, you can view the Ceremony of the Keys, but you'll need to plan that in advance. You'll need to write for permission at least one month before you wish to do it, tickets are free.
Admission to the Tower is around £10 for an adult. You can join a free tour by the Beefeaters who will take you around the site and tell you the history of the place.
I recommend going early before the hordes of tourists show up. It is a very popular tourist attraction.