This has literally been quite a process. I really had to push myself in terms of architecture here, and I believe there's still space for improvement here, but for now I'll call it a day.
I used some rock textures, but most of them got painted over again.
Feedback is very much appreciated! Enjoy.
Edit: Updated the sky, so the moon wouldn't be that much of a distraction anymore. Unlike expected it worked out pretty well, too.