Follow the girl inside and head west. There's no way to get past the pirate without being noticed, so ignore him. Turn north after you pass the plant. On the next screen, jump up the steps and deal with the pirates; one of them drops a strawberry. Take the lower path to the right; head south, dodging the fish. You'll come back onto the first screen above the cave's entrance. Don't drop down to the door; you can't open it, and you'll have to go all the way round to get back here (with the enemies regenerated). Instead, stay at the top and go right. The pirate here will drop a green apple. Now head north.
Just before the next screen, if you look at the shadow cast by the wall you'll see a gap. That signals a secret passage: go down it, you'll come out on a little ledge above the river. Throw the rock here at the wall for a heart.
You'll hear a discouraging voice as you enter the next screen, but ignore it. Go up first, not left (but remember this spot, you're coming back); jump over the three rocks and continue north. Jump up the steps and kill the plant, then jump over the gap; you'll need to use your Throw-Chain in midair to make it. Continue to the next screen, over the bridge, and smash the rocks: one of them contains a potion, the other a heart.
Head back to the junction where you heard the voice, and this time go west. There'll be a passage off to the north with three dragon statues inside. The rock at the bottom holds a chalice, but don't smash it: instead hook it and drag it up towards the statues. As you pass each statue it'll start bouncing and belching fire, but don't worry about that; press on and ignore any damage you take. At the top, jump on the rock you're pulling to release it, then smash the other rock for a heart. Now smash the rock you pulled up here and grab the chalice, and you can get back down through the fire without losing any of the health you just gained.
Next cross the bridge to the west with the plants on it, and head up the stairs. Stick to the right wall to avoid the boulders. When you reach the top, stay on the right, and you should be able to kill Rock Man without getting hurt. Go through the archway behind him, and head north then right. There's a door behind the waterfall; in this room is Rosen, who'll give you the news from Neugier and then lure the pirates away for you. So head right from the waterfall, dropping down. On the next screen you'll see the girl briefly. Kill the pirate – he'll drop some meat which will restore your health fully, and you'll probably need it. Then take the door to the north.
He'll drop the Gaol Key. Use it to unlock the door above you. The scene which follows should make your next move sufficiently clear.