This is the last image I created. The tower, the bridges (inspired by the shape of aqueducts) and the flying boat have been made in 3D in Blender. The rest of the picture are some photos that I have combined in Gimp.
What do you think of it?
It remember me the wizard of oz, I like the clouds, the flying boat and the background is great. I think the materials of the bridge and the floor can be better
I don’t think there are enough shadows in the image. You should also try using displacement on the stones and the bricks.
Thank you both for your comments, I will try to change a few things.