It is a very literal interpretational ballad of Napoleon in the moments before his death in exile.
The first three verses are where he recounts his former glory ("Old king is dead, long live the king") when he was going to rule the world, and how he held such power over his enemy. ("I used to rule the world/ Roll the dice") And then he tells of his downfall, when he sees that he cannot rule the world, and how he has become the lowest.("Now in the morning/ sweep the streets I used to own/ the walls were closed on me") It is very much like Napoleon's failed attempt to rule the world.
The chorus, which varies progressively, is a realization that he sees his own death. (Bells and Choirs would ring and sing, respectively, during a Funeral Procession, in some cases) "Mirror Sword and Shield" could mean that he is awaiting heaven, since the three are very Christian symbols. "Missionaries" is another Christian allusion, but appears to be useless, unless referring to that he is asking that a room in Heaven be prepared for him. "For Some Reason I can't explain, once you go..." appears to be a filler line, since it takes a different meaning later on. "That was when I ruled the world" is just a reprising line that sounds good. It restates that he no longer is glorious, and will die soon.
All the way from "Wicked and Wild wind" to "Oh who would ever want to be king" is a remembrance of his own rise to power and use of it. Also, because the French would Execute using the Guillotine, the "head on a silver plate" line also is an Allusion to Napoleonic France.
The second Chorus is the same, save for the line, "I know saint peter won't call my name". this is Napoleon being rebellious against death since he had been so powerful. Saint Peter calling a name is symbolic of dying and going to Heaven.
There is an instrumental bridge where I suppose you could say Napoleon realizes death is inevitable, and accepts it, while the music becomes stronger.
The final Chorus has the change of "I know Saint peter won't call my name" to "I know Saint Peter WILL call my name" Saying that Napoleon accepts his death. You can imagine Napoleon Closing his eyes at the words "When I ruled the world"
The song ends with a choir singing the main chords in harmony, in a sad, thoughtful fadeout, suggesting Napoleon is dead.
Those are the lyrical interpretations that I interpret from the song.
Me. (I'm making inferences, but this is the only way that it appears to me and feels right at the same time.)