In Cities XL, you build up your city in resplendent 3D from a selection of 500 buildings, whilst trying to keep your population happy - in the beta, players have managed to get up to 17,000,000 inhabitants in one city! You have to take account pollution, traffic, trade and class to build that perfect city.
Cities are built on amazing 3D maps (such as Mount St Helens), and the game really reflects that impressive scale, letting you zoom out to a Google Earth perspective or right into street level to watch your inhabitants going about their day.
If you want, you can go online and build your city on a planet with up to 10,000 other cities per planet. Here you can trade excess resources, chat with friends, specialize your city to corner your local market, or build Megastructures that aren't available in the single-player game.