Wow!
This song i feel can be directed at any king, obviously Coldplay was influenced by real kings....
"Now the old king is dead, long live the king!" - the king singing the song is describing how when he took power he obviously killed the last king and everyone rejoiced at him becoming king.
"And I discovered that my castle stand, upon pillars of salt, pillars of sand" - the king realized how his support was only because he held power.
~Favorite Line~ "Just a puppet on a lonely string, oh who would ever want to be king?" - referring to how his power was not real and he was only a figure head. He now realizes that the "king" was a false title and he had no power.