Good night!
I think I'm starting to learn how to use this. Instead of trying to prompt everything at one, I first prompted the background, then added a picture of a turtle and inpainted it to blend in style (not perfect, though). Then opened in gimp and deleted every shell part, added a city and again, inpainted to get the style. Then, another batch of img2img to blend the overall image a bit more and finally do the upscaling.