Complete game Liu Bei or Cao Cao on expert difficulty to unlock an extra battle for each side.
Play as Either Side
To play as either Cao Cao or Lui Bei forces, complete finish the game. Now at the main title screen, you will have the option to select either side when you choose New Game.
More difficulty settings
Beat the game with Liu Bei with normal difficulty setting. Beat the game with Cao Cao on the normal difficulty setting to unlock the expert difficulty setting.