Thanks to Wikipedia, http://en.wikipedia.org/wiki/Zwolle :
Zwolle is a municipality and the capital city of the province of Overijssel, Netherlands, 120 kilometers northeast of Amsterdam. Zwolle has about 115,000 citizens.
Archaeological findings indicate that the area surrounding Zwolle has been inhabited for a long time. A woodhenge that was found in the Zwolle-Zuid suburb in 1993 was dated to the Bronze Age period. During the Roman era, the area was inhabited by Salian Franks.
The modern city was founded around 800 A.D. by Frisian merchants and troops of Charlemagne. The name Zwolle is derived from the word Suolle, which means "hill" (cf. the English cognate verb "to swell"). This refers to an incline in the landscape between the four rivers surrounding the city, IJssel, Vecht, Aa and Zwarte Water. The hill was the only piece of land that would remain dry during the frequent floodings of the rivers. Zwolle was established on that incline.