[BrianG]

If terrorists (or anyone for that matter) want to get in, they will get in no matter what you do. But when they do, you just have to smack them down so hard that their buddies will think twice about doing it again. So no, physical walls are not the answer, but economic walls are (at least for the illegal immigration issue). Eliminate all the free handouts, jobs, medical, schooling, etc (basically erase any reason for them to want to come here), and the problem will take care of itself. Once all those people stop coming here, the other reason for illegal crossing (drugs) will be much easier to spot and deal with.