Final answer:
The era of European walled cities ended due to advancements in military technology, particularly the introduction of cannons and gunpowder, and the rise of nation-states that diminished the need for isolated walled defenses.
Step-by-step explanation:
The European walled city era came to an end primarily due to technological advancements in warfare and the evolution of political structures. During the Middle Ages, walled cities served as the stronghold of defense against various threats such as Vikings, Mongols, and internal rebellions. However, with the introduction of early firearms, cannons, and gunpowder, these fortifications became less effective. The invention of the cannon in particular allowed armies to breach city walls and castles, which previously might have withstood sieges for years. A monumental example was the fall of Constantinople in 1453, where Ottoman cannons overcame the city's formidable defenses.
In addition to military technology, the rise of nation-states in western Europe led to the development of larger political entities, which decreased the need for isolated fortifications. The increase in state power and consolidation of territories under stronger centralized authorities made large-scale walled defenses obsolete. Moreover, the development of new sea trade routes reduced the economic importance of many inland cities, contributing to the decline of their walled fortifications.
Conclusively, the military advancements and political changes during the 15th to 19th centuries marked the end of the era of European walled cities as they were replaced by new forms of military and political organization.