Tip: You can use Lunar Magic's Overworld Layer 3 Editing system to edit or create your own layer 3 images (castle bricks, fish, rocks) and apply unique effects per level via the Layer 3 Bypass in the main editor.
Contains two blocks that act as a midway point. One acts like a somewhat regular one, generating a sound and setting the midway flag. This one can only be activated from below (though you could move the labels around to change that). The other one should be used as an invisible midway point blocks - it doesn't give off any indication that you reached the midway point, but it does set the flag.