Thanks! I'm glad you liked it.
I took an abstract approach to the theme. The question "What's behind the door?" Implies to me something, ominous and perhaps unknown. A mystery, intrique and perhaps something entirely unknowable.
This reminded me of Plato's allegory of the cave where he describes in a narrative people who've only seen shadows of the real world and thus can't know reality as it is. So I went with that interpretation :D