Walls were normal in European cities. Then, during the Napoleonic wars, the French swept victoriously across Europe. In that phase, before their ultimate defeat, many of the fortresses of old were dismantled. It happened in my hometown Klagenfurt, it happened in Vienna and in so many more cities. The space was used for circular boulevards like Vienna’s Ringstraße.
Here in Győr, a part of the wall has survived.