One of the reasons I enjoyed Guardians of the Galaxy so much was because I got to explore a world in which I had no expectations. Take that crazy city Knowhere, which is apparently inside a giant Celestial head, I didn't even know it existed! Here's the VFX reel showing how the city was built.
Framestore, the people behind the effects, writes:
We built Knowhere, our most complicated environment to date, for Marvel's Guardians of the Galaxy and then staged a dog-fight that explored every inch of it. Here we've linked up all our shots in the 4-mile wide and 1.2 billion polygon world.
Here's some of their concept art for the city: