Cold Play - Viva La Vida what does this song mean?

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I love this song but I'm puzzled about the lyrics, is it about life and how fast it can take away everything from you? what do you think guys? 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 I hear Jerusalem bells a ringing Roman Cavalry choirs are singing Be my mirror, my sword and shield My missionaries in a foreign field For some reason I can't explain Once you go there was never Never an honest word And that was when I ruled the world It was the wicked and wild wind Blew down the doors to let me in Shattered windows and the sound of drums People couldn't believe what I'd become Revolutionaries wait For my head on a silver plate Just a puppet on a lonely string Oh who would ever want to be king? I hear Jerusalem bells a ringing Roman Cavalry choirs are singing Be my mirror, my sword and shield My missionaries in a foreign field For some reason I can't explain I know Saint Peter won't call my name Never an honest word But that was when I ruled the world I hear Jerusalem bells a ringing Roman Cavalry choirs are singing Be my mirror, my sword and shield My missionaries in a foreign field For some reason I can't explain I know Saint Peter won't call my name Never an honest word But that was when I ruled the world