If you're playing as Huang Gai, you will be tasked with heading east and going after the supply depot. Either of the other two characters will require you to head south and start capturing the bases around the area. Not long after capturing the second base, Taishi Ci will show up slightly to the west of it. Defeat him, remembering that he is slightly stronger than normal officers.
Taking out Taishi Ci will make the eastern and northern gates of Moling Castle open. This will trigger a mission to defeat both Ze Rong and Xue Li, so do that and continue going south. Taishi Ci will appear again, so get rid of him and then defeat the officer inside the next base.
Now, start heading east towards the main camp. Defeat any other officers in the way and then open the gates. Head inside and yet again, Taishi Ci will appear. Defeating him the third time will keep him away for good, as well as open the main gates to Liu Yao. Head inside and defeat him to finish the battle.