Bastions replace medieval walls
A city surrounded with modern fortifications in an irregular geometry. (e.g. Bergamo, IT, see above)
Over centuries, high towering city walls were said to be the best protection against any invader. Gun powder and canons, however, revolutionized the architecture of conventional city fortifications in the 15th century. Heavy ordnance needed higher and reinforced stations, free lines of fire and enough space for operating teams. Towers were reduced to bastions, smoothly growing earth walls replaced steep walls, and canons threateningly signalized the new defensive strength of many European cities already from far distance.
With the bastion fortress in Verona, built as of 1533, the Italian architect Michele Sanmicheli ranks as the inventor. A closed chain of polygonal buildings surrounded cities which were additionally protected by previously trenched moats. Especially impressive examples are located in Italy with Bergamo, Padua, Urbino, Treviso or Piacenza as we as along the Spanish-Portuguese border with Almeida, Valenca, Moncao, Olivenza or Elvas.
Even the Vatikan City, presently the smallest acknowledged state of the world, is a fortified city. The bastion fortress of Dresden, built as of 1546, is classified as model of modern fortress construction in Germany. The fortress town of Germersheim with its well-preserved buildings is one of the most outstanding in its category. The fortified city of Diest (BE) was not equipped until the "long" 19th century with a modern fortification and imposing citadel to protect the then young independent Belgium.
The Thirty Years War (1618-1648) dramatically accelerated the fortress construction with bastions. Kings, princes and cardinals competed with each other to protect their properties. The richer a city was, the more powerful was the fortress. In this way, Vienna owes its wealth to withstand Turkish sieges twice. Even today, the most European cities have eternalized their previous bastion fortification elements in the city layout.
At the golden age of fortified cities, the French marshal Vauban fortified more than 38 places and cities in this way for his King Ludwig XIV.