This is a book that manages to cover a huge spread of history without seeming to be thin in any part of the period covered. That is a major achievement. The author has also succeeded in providing an indexed collection of subjects that still read as part of long and unfolding story. Readers may be very surprised by just how important and enduring linear barriers have been and how they are likely to persist long into the future. A very interesting story that goes far beyond the technology of building walls.