Go east at the beginning and defeat Cao Mao, making him retreat to the northwest base. Head over there and defeat the officer guarding the door, then go inside and defeat Cao Mao once again. Now, go east and through the central gate, where you will face down an ambush.
Defeat the officer in the ambush and continue north again. When you reach the main gates to the throne room, you'll find them shut with no way of opening them. However, Jia Chong appears with a fellow officer and will unlock them from the inside. Go inside and defeat Cao Mao once more to finish the battle.