Successfully complete the game with the Lui Bei or Cao Cao.
Select "New Game" and you will be able to choose which side to be played.
After you finish the game as Liu Bei to unlock Cao Cao's side,
notice that the difficulty level is set at normal instead of
beginner (when you played as Liu Bei). Successfully complete
the game as Cao Cao under the normal difficulty setting to
unlock expert level. Select either Liu Bei or Cao Cao at the
expert difficulty setting. Complete the game under expert
difficulty setting to unlock a hidden battle for each side.
As Liu Bei, the hidden battle is The Battle of East Lau.
As Cao Cao, the hidden battle is The Battle of Chang Zheng.