There are walls

that are built to keep people out—the Great Wall of China, Hadrian's Wall—& there are walls built to keep people in—the West Bank Wall, the Berlin Wall.

There are walls that are built of stone, of timber, of steel, of concrete. & there are walls that are built with unthinking irony, as was that constructed by the organizers of a free U2 concert in Berlin celebrating the anniversary of the demolition of the Berlin Wall who built a two-meter high barricade around the event to keep it from public view.