In the beginning of the game, the Japanese Navy would attack your wonder in the north of your base. You shall try to defend it with the surrounding keeps (as you advance to the Imperial Age at the 1st second, your guard towers are automatically upgraded), and send your Fire Ships if necessary. If your wonder is destroyed at this moment, the Japanese fleet would immediately start attacking your base at the south. After you've sunk the Japanese fleet (which shouldn't be too difficult, with a few Turtle Ships on your side), you'll be given instructions to destroy their docks.