Clear two veteran levels (the topmost and then the leftmost battlefield). At the Map screen, push the joypad left as if you were trying to select a different battlefield than the one remaining. The highlight will disappear. If you then start, you will be on one of the two-player battle fields.
During the cannon stage, if you click five times on a place where the cannon can't be built, you'll go on ahead to the firing stage. One strategy is to place the cannon over a boat you plan to target and click five times, so that when you start the firing stage the crosshairs will already be over the boat.
If it starts getting a little hairy when you try to refortify your castle (ie. to cluttered with wall remnants), then try clearing those pieces away in the next cannon stage by shooting them.