If you stay in the water in the moat and kill seven ninjas with your sword, a blue creature will fly across the screen.
Catch him for a 1-up. This worked in all moat scenes!
Hold Select + Start + A + B on controller one and A + B on controller two. Power on the NES while those buttons are held.
Release the buttons when the title screen appears. Then, hold A + B on controller one and B on controller two.
While keeping those buttons held, press Start on controller one.