I believe this song is about Napoleon after he was exiled for the second and last time to the remote island of St. Helena in the Atlantic Ocean where he passed the remainder of his life in solitude with just his thoughts and writings to keep him company. This was a man who had ruled most of the "civilized" world now reduced to a memory.
I used to rule the world Seas would rise when I gave the word Now in the morning I sleep alone Sweep the streets I used to own
I used to roll the dice Feel the fear in my enemy's eyes Listen as the crowd would sing "Now the old king is dead! Long live the king!"
One minute I held the key Next the walls were closed on me And I discovered that my castles stand Upon pillars of salt and pillars of sand