Start the battle by heading east and going towards the two highlighted spots, which are reinforcement points. Take out the officer at both of these points to seal off reinforcements for Wei. With that out of the way, just continue advancing through the castle. If you're in the southern area of the castle, archers will appear you'll need to get on top of the wall to take them out.
If you are going through the northern area, you'll eventually come upon an area where arbalests will be activated. Ladders will be placed throughout the area, which will allow you to get behind the arbalests to destroy them. There will be sets of archers guarding them. Destroy all of the arbalests to make a gatekeeper appear, which leads to an arena-type area with Zhang Liao, Yue Jin and Li Dian.
Defeat them all and then head south. There will be a trap here, which are statues that spit fire, but they can't be destroyed by normal means. However, two Juggernauts will appear just outside of the room, which will allow you to destroy them. Once they're all gone, the gate to the next area will open.
In this area, you need to defeat both Jia Xu and Wang Yi in order to open the Outer Castle gates. Go inside and the gates will close, with an ambush appearing that has Xiahou Dun. With him out of the way, the main forces for Wei will appear in the next area, although the officers will appear before the main gate, who are Sima Yi, Cao Pi and Zhenji. Defeat them all to open the way to Cao Cao, who will be your last opponent in the battle.