The staircase divides Senta's two worlds. In the lower one we see her as a little girl, reading stories about the sea with her father. He and Daland are dressed identically and look very much alike, and it's clear that her obsession with the Dutchman is a reflex of her love for her father. The Dutchman's world is "upstairs," an inversion of the real world below. This makes sense as an idea--it's the only production I've every seen that offers an explanation for Senta's obsession--and as an image.