You need to withdraw with Liu Bei to Changban Bridge, however, Cao Cao's forces have decided to launch a pincer attack. This will cause everyone to stop withdrawing, and you'll need to defeat both Xun You and Cao Chun in order to start Liu Bei's forces to continue. When they get to the bridge, rendezvous with them to continue the battle.
This will cause some of Cao Cao's forces to charge towards the bridge, so some of the garrisons will now be open. However, you get a report that Adou, Liu Bei's son, has been left behind north of the village, so you will need to rescue him. When you get there, defeat the two officers and find the handmaiden to rescue Adou, then return to Liu Bei. On your return, you'll be ambushed in the eastern fort, so defeat the officer and then return to the bridge.
Liu Bei will resume his march, but is eventually blocked by an ambush. Don't go towards it, but stay at the bridge and repel the forces that will now be moving towards it. There will be two waves of enemies, with the first consisting of Li Dian, Yue Jin and Xu Zhu. The second wave consists of Xiahou Dun, Zhang He and Xiahou Yuan. Once you defeat the second set, start heading towards Liu Bei along the path chosen for you.
Once you get close enough to Liu Bei, another ambush will appear, with two ballistas and an officer. You need to take out the ballistas as soon as possible to minimize the damage to the peasants, then you can worry about the officer. With them gone, jump down and then Zhang Liao will appear, so take him out to make Liu Bei pass through the gate and finish the battle.