Use the scroll bar to see the entirety of this map.
Once you enter, leap over the gap on the right, and shoot out the sentry cannon by the gray bricks. Jump into the alcove, and then press down to squat. While squatting, press forward to crawl through the narrow gap. As you leave the first tunnel and approach the second, another sentry will be inside shooting at you. Remove it as quickly as possible. In the next space between tunnels, you should find a heart.
Once you reach the other side of the bricks, you will see a floating platform. In order to reach the far side of this stage, you must hop from one floating platform to another. While this can be done, it is a little tricky and tedious. If you miss a jump, you will fall down to the cavern below, and be forced to repeat the stage once you escape.
If you happen to have a Jet Pack on you, this stage presents one of the best occasions in the game to make use of it. If you don't have one, and would like to buy one, call the store down before you proceed. (If the store is sold out of Jet Packs, make sure you buy another Store Call, and call it again until it does have Jet Packs.)
Whether you leapt across all the platforms or simply flew to the right by Jet Pack, you reach a stretchx of land (be sure to unequip your Jet Pack when you land.) A single high-bouncing enemy occupies the area. Proceed to the right, through the narrow gap in the gray bricks, and you encounter the fairy Faysha. She tells you that a devil named Rowdy has kidnapped a hermit on Route 35. To help you get there, she flies off to open a passage that leads you there from Route 6.
Collect the two hearts and then return to the left. It actually faster to escape this route by intentionally falling down and escaping in either direction through the cavern.