I read this book 20+years ago, and even then it seems to have been an old book. I really wish I could remember more details. The land was divided into large sections by aliens, which was kept isolated from each other by some invisible barrier. In each section there was setup different era's of our past, so that aliens could visit and watch (for entertainment if I remember correctly). The protagonist was a man who escaped his section and, after gathering a few friends (a girl?) and visiting a few different sections, was able to disable the barriers.