Wonder Boy in Monster World/Nightmare Castle

Once you have the Legend Sword head to Purapril and climb the tower to reach the queen. Head to the far left and press up under the belltower to be teleported to the Nightmare castle. Get the Heart from the chest, then continue up the steps to the castle itself. The castle is a big maze and you have to choose the correct direction at each point in order to proceed; if you go the wrong way you'll end up back at the start of that part and will have to try again. Follow these instructions to get through:


 * 1) Fall down the first opening.
 * 2) Walk to the right and climb up.
 * 3) Again walk to the right and climb up.
 * 4) Go to the left until you can get up.
 * 5) Go to the right until you can get up.
 * 6) Jump on the platform with the wheels and jump off when you're near the chain. Mind that the Legendary boots are located in this area.
 * 7) Jump on the rope lifts and fall into the gap, there you'll find the Armor of Legend. Enter the door to get up.
 * 8) Go to the right.
 * 9) Grab the first rope to get the Shield of Legend, then grab the second one and enter the doors.
 * 10) Defeat the two skeleton knights, their feet are vulnerable. They take 16 hits each with the Legend sword.
 * 11) Defeat the dragon, 11 swings with the Legend sword will finish him.
 * 12) Keep jumping on the two small platforms to reach the platform with the small snake, then continue the progress.

Each of the three heads take 12 hits with Legend sword, or 6 if using Power magic to knock them off the main body. So 36, or 18 hits in total using Power. Once you've defeated the Almighty Demon King, the Prince of Darkworld will now appear.