Two player game in single player mode:
Complete two veteran levels, such as the topmost and the leftmost
battlefields. Press Left at the map screen, and the battlefield
highlighting will disappear. Begin game play to start on the two
player battlefields.

Quick targeting:
Enter the cannon stage. Attempt to place a cannon five times at a
location that cannot accept a cannon. The firing stage will begin
with the crosshairs directly over this location.

Clearing old castle parts:
Clear old castle wall remnants by shooting them during the cannon
stage.