An undefended wall is no defense.
Let me just note that building a wall along the Mexican border is not a defense against people (migrants, drug dealers, terrorists, or other criminals) coming into America. A wall can only slow down people to give time for defenders to react to the penetration and stop the people before they can get too far.
If the wall is built, will we have adequate border personnel to defend it?
And how will those who would cross it react by coming in elsewhere?
Fixed fortifications may not be a monument to the stupidity of man, but viewing fortifications as the only thing you need to stop a foe surely is.