Short answers, pulled from the story.
The proto-city of Jericho in the West Bank had a wall surrounding it as early as the 8th millennium BC. This structure marked the very first time humanity chose to enclose itself against the unknown. The wall was built of stone and stood nearly 4 meters high.
The eastern wall of Ancient Linzi reached a maximum thickness of 43 meters. This immense thickness was a deliberate strategy to absorb the energy of siege engines and gunpowder artillery. Chinese walls were constructed using wooden frameworks filled with layers of earth that were tamped down to a highly compact state.
The Florentine engineer Giuliano da Sangallo compiled a comprehensive defensive plan using the geometric bastion and full trace italienne. This design became the dominant defensive structure in Europe by the 1530s. The star fort featured angle bastions that allowed defenders to fire parallel into the opposite bastion's line of fire.
The Berlin Wall was built from 1961 to 1989 by the German Democratic Republic. It was constructed to prevent citizens from fleeing to the West German exclave. The wall eventually fell in 1989 after nearly three decades of division.
The walls of Avila in Spain and the walls of Tallinn in Estonia are not just remnants of the past. They are living monuments that continue to define the identity of the cities they protect. These walls were often a manifestation of the pride of a particular city.