A city wall is a defensive structure built around a city to protect it from invasions and attacks. These walls were often made of stone or brick and could be several feet thick. Many ancient cities, such as Jerusalem and Rome, had impressive walls that served as barriers against enemies and helped control access to the city. In addition to their defensive purpose, city walls often included features like towers and gates. Towers provided vantage points for guards, while gates allowed for controlled entry and exit. Over time, as warfare evolved, many city walls became less important, leading to their decline or repurposing in modern urban areas.