I was reading an historical novel and one of the most important developments was that they found a city with no walls in a northen part of the world. The protagonists find this incredible rare, since it means that the town is defenless from an invading army (which they use scores of mercenaries, which was kind of a bad idea), yet they find it allow the town to had greater health and capacity to grow as an economic center.
So I got wondering, since when or what forced towns and cities to go from walled to unwalled and was that a massive reform in nations or did this changes happened through the centuries one city at the time?
We can very broadly divide this up into periods. First, in the 14th c. , with the advent of gunpowder in Europe, there was inaccurate artillery. Destructive, but not always crucial. In the 15th c. gunpowder and cannon were improved and there was accurate artillery. In large part because this accurate artillery was used effectively in the Italian Wars, going into the 16th c., fortification got more complex- those medieval curtain walls were replaced by pointy bastions, tall keeps replaced by redoutes. Cities that tended to see armies attacking them, like Vienna , went from smooth walls to pointy ones. In the following century, fortifications would become even more elaborate, with points added to the points, bastions in front of bastions. The master of the science fortification at the end of the 17th c. was Louis XIV's general Vauban, whose forts are still found in France.
In the 18th c., there was some equality. The besieging army would start digging zig-zag trenches up towards the walls. If they were successful in getting reasonably close, the army could then set up artillery and , given time and ammunition, the fortifications would be breached. But the fort or walled city could often keep that from happening, either with sorties, artillery fire, or with the arrival of a relieving army.
After the mid 19th c. however artillery gained in range and accuracy. It was no longer necessary to get cannon pretty close, in order to breach a fortification. After the defeat of Napoleon and after Bismarck had played his "combinations" to gain territory and unify Germany there was also a pretty long period of stability and peace. Cities and commerce grew, and the walls got in the way of that- got in the way of expanding roads, transportation, neighborhoods. The big walls seems pointless. Vienna would convert its wall to a boulevard, the Ringstrasse, in 1865. Cities in Belgium, like Antwerp, would make similar changes around the same time. Though the picturesque fortified gates were sometimes left intact, earning the gratitude of many photographers and tourists.
