As you leave Eagle Tower make sure you jump off the lower RIGHT ledge. Trace your way back to the river on the Mountain Range. Go up the stairs you went up to get Eagle Key. But instead of going in that cave, go up the stairs and head dur west (left). Use the Hookshot to make your way across the broken bridge, saving Marin in the process, and stop at the lone bush. Cut the bush and go down the stairs. Make sure you bomb that lower nook and get the Heart Piece! Exit the cave and equip your shield. Make your way up the hill, blocking the bouldders, and jump off the ledge. Bomb the nearby cracked wall and go inside. Block the fire blasts with your shield and go up the stairs. Make your way left and past the Telephone Booth to Turtle Rock. Play the Frog's Song of Soul in front of it and... fight it... When you beat it the door will open and you can enter the dungeon.