When I first heard this song, I thought of Napoleon. "I used to rule the world" could be referring to the fact that Napoleon did, in fact, eventually conquer most of Europe. "Now in the morning I sleep alone" could be about his exile on Saint Helena after his reign as Emperor. The part about feeling "the fear in my enemies' eyes" could be about how he was so powerful and everyone in Europe feared him, especially those under his rule. "Now the old king is dead! Long live the king!" could be a reference to the death of Louis XVI and the French monarchy and the subsequent rise to power of Napoleon. "One minute I held the key, next the doors were closed on me" could be about how he essentially ruled most of the civilized world, then met his defeat at Waterloo. The part about the castles standing "upon pillars of salt, and pillars of sand" could be about how he thought he was unstoppable, but then everything fell to pieces around him. The "wicked and wild wind" could be a reference to the French Revolution, which "blew down the doors to let me in" (basically giving Napoleon the chance to seize power.) The Jerusalem bells and Roman cavalry choirs could be an indirect, religious reference to Napoleon's feud with and then capture of Pope Pius VII. "People couldn't believe what I'd become" could be about how Napoleon rose from being a soldier all the way up to Emperor and arguably the most powerful person in Europe at the time. The part about "revolutionaries wait for my head on a silver plate" could be about the continuing political turbulence in France throughout Napoleon's reign, and how many were unhappy with the way he ruled.
However, I have also heard people say it's about Louis XVI, which I could understand. I would have to say whoever it is about would most likely be French, as referenced by the French flag on the album cover. But I don't know. Even if it isn't about anyone in specific, it's still an excellent song about power and the rise and fall of a leader. Let me know what you think!