When the castle has been destroyed, you get a ship and a dock nearby. They should be sieging the castle, not fighting enemy Chu Ko Nu. You'll encounter some enemies, which you shouldn't have trouble dealing with, but keep your pikemen out of the fight. Once your two Old Han armies are out, you should head west to destroy the castle. Keep building villagers, you'll need to have a powerful economy. Remember to unload the Wokou Horsemen from the ship to join your main army, though. Due south from your dock is an enemy dock, which you should also be able to destroy. The junk will be able to take out the outlying ships. Choose the 600 Gold shipment, collect it and send the Wokou Junk.

In the meantime, start researching economy upgrades at the market. Once the war academy has been built, build two Old Han Armies. As your first shipments, choose Chu Ko Nu and Qiang pikemen. After that, you should build a war academy and a market. Captain Huang's Heavenly Fire can take out multiple units from distance, whereas Lao Chen will have to close in. While fighting, use your two heroes to kill larger groups of enemies. Don't waste your units on the shore, trying to take out the enemy ships - your town center is more than capable of taking care of them. You'll be periodically attacked from the sea and on land, so don't stray too far to begin with. Gather the nearby treasures with your army. Put the town center gather point on wood and put the rest of your villagers to cut wood as well. Have your town center producing villagers constantly. Start with moving most of your villagers on food - there's a herd of animals southeast of your camp.

In the beginning, you have a sizeable army, but don't be lulled into thinking you can defeat your enemy so fast.

Age of empires 3 campaign