At night in a city, virtually any city, the scene before you changes dramatically. In some locations the architecture and spirit of the city is merely enhanced, shown with a different, though recognizable look.
In other locations, the city at night transforms into a new place, sometimes brighter and more adventurous, sometimes more alive and mysterious.
Either way, making images of cities at night can allow you to often capture a “new city” with a different life than it has during the day. Making photos at night has its own challenges, but it's still subject to the same set of constraints as daylight photography; aperture, shutter speed and light sensitivity. The difference is, these constraints often push you and your equipment “to the edge,” compared to photographing the same city during the day.