AWA420 - What were Roman walls for?

Russ asks, "I don't know what it is called, but at a certain point ( or a certain number of points) in time, Rome decided to build walls to defend their empire. Most famous, of course, is Hadrian's Wall. But of course they are many more, such as the Antonine Wall, Walls of Lugo, Aurelian's Wall and Londinium Walls, to name a few. These walls were constructed differently and had different purposes.…