This video is of the Sky Tower in Dragon Quest v SFC. This is known as the stairway to Zenithia in other versions of the game.
At the top of the tower, the hero learns that Zenithia is no longer up in the sky, but has fallen into the ocean. He receives advice for how he can find the sunken Zenithia and perhaps figure out a way to raise it into the sky again.
The Magma Staff in the chest at the top of the tower is a key item which is needed to continue on in the game.
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