Simple, through up an unedited city skyline and have people submit their guesses, maybe ad a few rues and hints as you make the cities more challenging. Or perhaps blur the skyline or delete some pices to make it more challenging. I also am a huge fan of the worlds most impressive skylines. I am currently living in one of them, it contains a building built around money an oyster of the sea and lies on a famous tributary, can you guess my skyline?