Lumiose City is the central hub of Kalos. It is a booming, sprawling metropolis with skyscrapers as far as the eye can see, as well as a tower in the center of town resembling the Eiffel Tower in the real-world Paris, France, which is where Lumiose City is based off of.
Lumiose City's Location in Kalos
Above is a look of where Lumiose City is on the map. This is its confirmed location in the Kalos region.
Pictures of Lumiose City
Here are some pictures and screenshots of Lumiose City. You can click on any of them to view a larger version.