How about a Climbing City?! Where, instead of walking around, driving, or flying little ships, the inhabitants (whether human, alien, or creaturish) got around places by climbing to them. This would open up interesting, new, dimensions to the spatial conception of a city. The idea would be that because of geography, security (look up Meteora, Greece! - or various monasteries in the mountains), or the lack of space (pop density), the city began expanding vertically (or even upside-down), and people began climbing from day to day to get to their jobs, school, parks... etc. Think about it! Would the inhabitants need safety equipment or would they just all become bada$$es? Or maybe it is an alien world and the creatures prefer climbing over anything else!!!