One of the earliest human discoveries must have been how to build a defensive wall. Archaeologists, looking for traces of ancient human occupation, always look first for signs of stone walls, or wooden palisades, or earthen mounds thrown up to keep those inside safe from those outside. It seems that we were even less neighborly in prehistoric times than we are now. Walls have always been popular. In Europe in past centuries every city and town had a wall with guarded gates, which were often difficult for a stranger to pass, and were closed at nightfall. When the plague was abroad in medieval Europe the cities threw out all foreigners and shut the gates, because somebody had to be blamed for the infection. The citizens meanwhile locked themselves in their houses and wore cloth masks around their faces. So nothing much has changed in five hundred years. You can still see the remains of those city walls in many places, as well as impressive fortresses with walls up to forty feet high,