Well, well, let's look at The Great Wall of China, shall we? You know, the one built sometimes atop dizzying cliffs, so big you can see it from space...the Only thing on earth visible from space, in fact. 16'- 25' of solid masonary wide, 26' tall at the tallest part, 5,500 miles long, and fully manned its' entire length. I mean, we're talking a WALL here!
Total number of barbarian hordes it kept out of China: ZERO. Let that sink in!
Do I need to say good luck to a 10' wall on flat ground (relatively speaking)? Or are you an idiot?
Mexicans are resourceful. They will find a way either over or under the wall.
@RobertMartin desperation leads to inspiration in many cases. I have read that tunnels are the overwhelmingly preferred method, and have been for some time. Maybe we should plant bamboo?