During his campaign for the presidency, one of Donald Trump’s signature promises was that he would build a “great great wall” on the border between the US and Mexico, and Mexico was going to pay for it. A year and a half into his term, with only a few prototype segments erected, the wall is the 2,000-mile, multibillion-dollar elephant in the room of contemporary American life. In The Great Great Wall, architectural historian and critic Ian Volner takes a fascinating look at the barriers that we have built over millennia. Traveling far afield, to China, the Middle East, Europe, and along the U.S. Mexico border, Volner examines famous, contentious, and illuminating structures, and explores key questions:  Why do we build walls? What do they reveal about human history? What happens after they go up? With special attention to Trump’s wall and the walls that exist along the US border already, this is an absorbing, smart, and timely book on an incredibly contentious and newsworthy topic.