Most of the 9-11 terrorists came in over the even more unprotected Northern border.
I'm not familiar with the entry methods of the 9/11 19, but I live here on the US/Mexico border and know full well what's going on down here.
That, and the physical proof I offered, are the reasons for my comment.
I am not qualified to comment on the Northern border and tend not to comment on things of which I am ignorant.