I think the biggest problem here is that the render doesn't blend with the background. You just made it b/w but the white mist/clouds/smudging/whatever makes it worse since it's kind of seperating the render from the background. Also to add some depth and creating a better focal, some blurring of some parst of the background is more effective than just sharpening the whole thing...
Well, my attemp on CnC... Keep improving man.