What is the reason for the difference in the thickness of European traditional residents' city walls?

This is caused by climate differences. In the coastal areas of western Europe, the climate is mostly temperate and maritime, characterized by mild and humid, and the annual range and diurnal temperature range are relatively small, so the thickness of residential walls in the coastal areas of western Europe is generally slightly thinner. However, under the influence of temperate continental climate, the annual range and diurnal range of temperature in the inland areas of Europe far from the ocean are relatively large, so thick walls can play a good role in heat preservation (at night) and heat insulation (at day). This is the same as the wall thickness of the residents in the east and west as I said. . . .