1-up!
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!

Ten lives
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.