A wall is a vertical dividing surface. It divides space in buildings into rooms or protects buildings. It is usually made of stone or brick. Walls have two main purposes: to support the top part of buildings, and to divide space, giving protection from invasion, escape, and weather.
Before powerful artillery was invented, many cities had protective walls. Since they are not suitable for defense any more, most city walls have been removed.
The term "the Wall" usually referred to the Berlin Wall, built during the Cold War, which fell in 1989, but may also refer to the Pink Floyd album of the same name.