|After clearing this battle, you will be on the Historical route for Shu's story. For the beginning of the Hypothetical route, go to 9W - Battle of Fan Castle.|
At the beginning of the battle, just concentrate on clearing the area of enemies right outside the castle. Whenever you see that Wei's flood attack has succeeded, you'll be told to head to the escape point in the southeast corner. On the way there, an ambush will appear and you will need to defeat Niu Jun to make every continue withdrawing.
Around this time, Guan Ping will die in battle and the victory conditions will change. Either way, continue to the southeast corner, where a bunch of Wu officers will appear, including Lu Xun and Lu Meng. If you have the Rage meter filled, now would be a good time to utilize it. Note that while you're fighting here, the others will likely have begun moving to the west, as a new escape point will appear in the southwest.
The only problem is that when your allies reach the gate, it will shut and Mi Fang will betray you. In order to open the gate, you'll need to defeat both Mi Fang and Fu Shiren. You now need to start advancing to the north and get to the western checkpoint to escape. On the way, Wei will have an ambush waiting, which includes some ballistae and some officers approaching from the south.
Defeat them all so you can continue, when Xiahou Dun will appear and shut the northern checkpoint gate. Get rid of all the officers that have shown to open the gate, then continue southward. Cao Ren will have sealed off the escape point, so take him out to put an end to this battle.