Apparently you haven’t keep up with technology — airplanes, tunnels and ladders. Besides 99% of the problems flow through legal ports of entry, as tens of thousands of folks pass back and fourth across the border every day.
If your big on walls why not visit the great walls of the world. I hear there’s a really big one in China. Or perhaps you might want to buy a small island and build a wall there. I hear isolation is wonderful for malcontents.