From a start door, break a fragile iron box next to a door and go down to the bottom of the stage.
Step down from the floor where a door is.
When you get to the bottom, go toward the left.
You will see a wooden box and four iron boxes there.
The bottom iron box among them is a fragile iron box so break it. But don't break a wooden box beside a fragile iron box.
Climb iron boxes of three blocks height by using a wooden box as a step.
There is a tall pile of wooden and iron boxes on the left edge.
A key is hidden inside the top fragile iron box of them.
Break the iron box to get a key.
Then move toward the right.
There are appearing iron boxes next to a goal door so arrange a wooden box to go up and enter a goal door.