The Cross reveals invisible Moas; six placed Crystals release the Great Palace barrier.
Optional
Link Doll #4 is in the northwest corner of the swamp south of the River Devil. Palace point bags are optional.
Missable
Link Doll #5 becomes unobtainable once the Hidden Palace seals. Collect it in the late boss approach before fighting Barba.
Numbered 100% route
South of Nabooru, play the Flute before the River Devil. Cross the three forced rock-thrower scenes and collect Link Doll #4 from the northwest corner of the swamp above the graveyard.
Explore the northern cave and eastern forest. Along the southeast shoreline, enter the forced scene and collect Heart Container #4.
Return through the forest cave and strike the correct forest square with the Hammer to reveal Hidden Kasuto.
Show the three earlier Magic Containers to the old woman and enter her house for Magic Container #4. Find the Wise Man's basement and learn Spell.
Cast Spell at the far end of town to raise the hidden building, then enter it and claim the reusable Magical Key.
Return west through the swamp. Play the Flute at the center of the three rocks in the desert to reveal the Hidden Palace.
Inside, search the first left and right branches for point bags, then descend and use the Magical Key on every locked door.
Continue east past Wizzrobes, invisible pits, and block walls. Defeat the Iron Knuckle guarding the Cross and collect it.
Backtrack to the invisible hole, follow the collapsing-bridge drops, and use Fairy where the lava gap requires it. Preserve magic for the boss approach.
Before dropping to Barba, collect Link Doll #5 beyond the Blue Iron Knuckle and two Bubbles. Defeat Barba and place the sixth Crystal.
Boss strategy
Use Jump to control your arc between the three platforms and stay out of the lava.
Barba's exposed face is the only target; slash as it rises, then reposition before the fire breath.
Do not cast Fairy in the boss room—the form cannot attack Barba.