It is the oldest of palace/fortress built in Londo...

It is the oldest of palace/fortress built in London, by William the conqueror, thats why it is not that huge, but being old it is connected to a list of historical events and many bloody executions.
Best part about this place is of course, display of crown jewels. That is something you would not see anywhere else. Like Kohinoor diamond, and the biggest one which is close to 500+ carats.
You would need 4 hours, or 3 hours with bit of rush to complete this place. Try to visit crown jewels, later, as queue will be much smaller.