Pretty good though also pretty lazy-looking. Eh the backdrops that are single colors seem like they could have been done a few minutes each. I know because I've done that myself. You get good effect for little effort so I don't hate this technique. Just maybe not the thing to show off I guess.
A few advices:
You have 4 backdrops and I think you could merge the first 2. There's a sorta offset here, the turrets are kinda the same size on all layers, ruining the depth.
The colors only differ in brightness while the hue and satuartion seems the same. This makes it more flat-looking I think.
Having single color layers is good enough for backgrounds, espacially wokring good with fog or forest areas. Howevver you make yours the focal point of the picture.
You have some nice clouds. I think, personally, it would look pretty cool if you put some clouds in front of the final fortress layer, makes it more busy and life-like and gives it a bit more scale.
Almost looks like a beautiful warship that has been beached. Beautiful stuff.
Do edit the detail file to x1 resolution for people to enjoy. There is a built in zoom function on-site.
I like how 1 color planes works here - simple but strong composition