In order to clear the stage, you need to defeat all of the King Ghosts and Skeletons.
Originally, there are no Skeletons on the field, only a set of King Ghosts lurking at specific points on the map and a continuous spawning of Porcupines.
The Porupines will continue to respawn automatically until the King Ghosts and Skeletons are gone.
The King Ghosts are large enough that, for most of them, you can simply hack away at them with your sword at every opportunity they give you.
Others, however, will tend to float over the water, making it necessary to use magic to defeat them.
Take note that there are only 2 bridges in this stage.
Enemies
Skeleton
The Skeletons here are the same as the ones anywhere else, except you do not need to use elemental damage to defeat them.
Porcupine
These little critters will run around and occassionally jump at you.
For the most part, they just get in your way.
If you run along the railing, you won't bump into any
King Ghost
These are similar to their lesser form, but in addition to being able to launch fireballs at you, they can also summon Skeletons.
Secret Treasure Hint
Heart of mercy.
Obtaining the Secret Treasure
You'll definitely want to put on the High-Speed Boots and maybe even the Shuttlecock Boots.
The idea here is to get to the end of the stage without letting a single enemy die, either by you or by falling into the water.
Stick to the side of the wall on the upper level and make your way down.
When you reach the end, jump off from the upper level to the lower level and again to the wooden platform marking the exit.
If you managed to get to the end without any enemies dying, the chest will appear nearby.
Reward: Money Crest (Doubles cash from enemies)