This heart is located in a hidden castle, so the first step is to find the castle.  To start, follow the path from the gate all the way to the Hall of Heroes.  Right before you reach the entrance, look for a ledge to your left.  Follow it behind the waterfall where you will find a climbing wall. 

Proceed up the wall and then jump over a gap when you get to the top.  Cross the bridge and then hop onto the rock to cross the river.  On the other side is another climbing wall.  When you reach the top, look for a cave behind the waterfall.

You'll find a couple of doors - go through the one straight ahead.  When you see the tile floor, jump on the dark purple ones as the light ones break. 

Now watch for some airs jets - they will reveal an invisible platform.  Wait for the jets to turn off and then jump on the invisible platform and then across.  When you reach the tilting platform, stand on the close end to to raise the other and then hurry across and jump off the end.  The next invisible platform is too far too reach if you let the tilting platform drop. 

You'll next have to go along a ledge, but watch for sir jets that will push you off.  When you reach the end, look for another invisible platform.  From there climb the pole you'll see.

Move along until you reach some moving platforms.  Make your way and you'll reach a walkway with dropping blocks.  It's easy to get past them by watching their timing.  Watch for Shreeks and if you wake them up, wait for them to fall back to sleep before running past.  Next it's more moving blocks.

After you pass another Shreek, you'll find a rotating block with jets.  When you make it past, jump right away and keep going because the platforms will drop away.  You'll reach a pole that you'll need to climb.  As you move up, watch for a jet and time your way past it.  Jump from the pole to the stairs and when you reach the top you'll find the heart.