Paris: The Parisii (or Quarisii) were a Celtic people who lived on the banks of the Seine from around 250 BCE. In the 500s CE the Frankish chieftain Clovis made Paris his capital. In the 800s Paris was an important trade center, made so by the river Seine. Paris declined during the Viking invasions, but in the late 900s Hugh Capet made it a prestigious center again – its population around 20,000. In 1163, the Notre Dame cathedral was built. In the 1200s Paris had a population of around 110,000, the University of Paris was founded and its first sewer system was built, consisting of open troughs that ran down the center of each cobblestone street.