Human beings have known the defensive effect of city walls for a long time, and I do not know how many offensive and defensive battles have been staged through the ages. When the Mongols began to fight the world, the nomadic cavalry regiment was at a loss for the siege. However, the Mongols were good at learning and soon mastered the tactics of attacking the city, and the comprehensive use of siege weapons from various places achieved very good results, and many impregnable cities were destroyed. The thinner walls in Europe at that time could not resist the attack of the Mongols at all, especially when the Mongols began to use a variety of powerful firearms, which were more destructive. From then on, due to the improvement of artillery firepower, the role of city walls gradually disappeared from human history.
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How the city wall withdrew from the stage of history