Sunset

Well, first off let me tell you that the idea for this is truly incredible.

Put more effort into the planet.
The walls and oher painted stuff looks like a good basis for TONS of detail that you could add.
Add tiny little details in that part under the arch. That could be a city far away...
Stars? Add them up top. ^.^

I really think this piece has so much potential it is incredible, just never stop adding detail to it, it could become something of pure wonder.