Humans have only known the defense effect of the city walls for a long time. From ancient times to now, I don't know how many offensive and defensive battles have been fought. When the Mongols began to fight in the world, the nomadic cavalry regiments who were good at the field were helpless against siege. I quickly mastered the siege tactics and integrated the siege weapons from various places to achieve a good result. Many sturdy towns were destroyed. The thinner walls of Europe at that time could not resist Mongolia. Human attacks, especially the Mongols, began to use a variety of powerful firearms, which were even more destructive. Since then, due to the improved firepower of artillery, the role of city walls has gradually disappeared from human history. The text is intended to explain:. ['How the Mongolian Corps conquered Europe.', 'How the city wall exited the stage of history.', 'How the firearms played a role in the siege.', 'How the Mongolian Nomadic Regiment adjusted its tactics.']