It will need maintenance though, as it will be continuously attacked. And what is absurd about an unguarded wall being very ineffectual? As an obstacle it repr

It will need maintenance though, as it will be continuously attacked. And what is absurd about an unguarded wall being very ineffectual? As an obstacle it represents a rather modest practical challenge compared to walking the length of Mexico for example. A tall enough ladder isn't a very exotic object, and rudimentary training of its handling will be provided by the same people who teach how to cross the desert. So putting up a wall on some parts of the border makes it not worth the the effort and risk of crossing at those parts, but if you put it up all over the border you only modestly increase the total risk.