The mayor of the Texas town Trump is visiting tomorrow made an very interesting and largely overlooked point today: if Trump builds a wall along the Rio Grande where it forms the border and doesn't put it in the middle of the river, then that wall serves no purpose. As soon as anyone on the other side sets foot on land, including on the other side of the wall, they are legally in the US and so if they claim refugee status, they must be processed. Which would not necessarily keep them out of the US if Trump succeeds in keeping them in Mexico while their claims are processed, but I think that remains to be seen, and of course it would just add to the backlog of claims and administrative chaos.
As someone who lived and worked many years in Central America interviewing hundreds of people applying for refugee and other kinds of humanitarian status, my view is that Trump would make America much safer by spending a lot more money supporting the economies and especially law enforcement in the source countries so that people there had less reason to try to come to the US. While that would not solve the problem - nothing will "solve" it of course - a little money would go a long way to making it better.