Select either Liu Bei or Cao Cao, and successfully complete the game on the 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.
Successfully complete the game with Lui Bei and Cao Cao. Select "New Game" and you will be able to choose which side to be played.
Extra difficulty settings
Successfully complete the game with Liu Bei to the normal difficulty setting. Successfully complete the game with Cao Cao on the normal difficulty setting to unlock the expert difficulty setting.