Why was the medieval city called a craft center?

Crafts are closely related to trade, because you need to put your products somewhere. Moreover, cities often specialized in some kind of craft (blacksmithing, clay-making, marble work, etc.). Cities, as a rule, were built on the banks of a river or sea, so it was easier to sell products somewhere. Plus, the city has more population and a wider sales market, a higher level of well-being. The city was surrounded by villages and castles of feudal lords, therefore, it was possible to sell the products of artisans and buy food there.