Before we examine the merits of this idea, it should be known who is making money on the wall. The latest design of Trump's 'wall' (which he acknowledges is a barrier more than a wall) includes high steel slats with spikes (the better to impale people and birds with). Some estimates have the slats costing upwards of $24.5M per mile (for a 1,993 mile border that logistically cannot be contiguous). That's a lot of money, especially with the steel tariffs that Trump has demanded.
Who stands to make a profit from the sale of the steel? Unsurprisingly, a Russian steel company whose biggest shareholder is an oligarch and Trump family friend. The company, Evraz North America, supplied Keystone from its steel plants in Canada. The oligarch who owns Evraz, Roman Abramovich, is a very close ally to Putin (owing much of his significant wealth to him) and a personal friend to Trump and the Kushners.
One of history’s greatest mass murderer understood something that apparently Trump does not. Stalin supposedly once said, “Ideas are far more powerful than guns. We don't let our people have guns. Why should we let them have ideas?”
Will the wall stop harmful ideas from entering America? No. As stated by John B. Alexander, you must realize that the most dangerous and virtually unprotected port of entry for terrorism is the human mind and no wall is going to prevent ideas from entering. In Parameters, the Journal of the U.S. Army War College, Lt. Col. Timothy L. Thomas explains that the concept of “‘information warfare’ falls short when the individual soldier, not his equipment, becomes the target of attack.” What is depicted here is that the targets for ideological persuasion are far wider than members of the military but the effects just as devastating.
In other words, WALLS ARE USELESS TO STOP IDEAS FROM ENTERING A MIND.