Nice start of a concept there.
It looks like you will be using lots of atmosphere and fog right off the bat.
When I have built foggy matte paintings at work I generally have to create the matte painting without the fog first and then add the atmosphere separately. These separate layers get provided on to the compositor who can then turn up or down the quantity of fog based on what notes come in from the client.
My suggestion is to start collecting photo reference at this point and try to block in the painting without using too much atmosphere from the start. Remember as you work to keep the atmosphere separate.
I realize this isn’t for a compositor down the pipe… it’s for the forum competition, but the matte painting file becomes a lot easier to work with if you keep things like fog and thick atmosphere in separate layers and don’t bake it onto your objects.
Looking forward on seeing progress on this 
Also… I realize it’s a concept, but be careful of your perspective if you’re re-using the plate elements for other parts of your composition.
You might want to build a grid of some kind going to your vanishing points and then cut up your architectural reference into separate planes and then piece it back together via the guidelines of the grid.
It will help the perspective in the final.
