Mankind has put people in space. Even countries without space technology can certainly get over (or under) a wall. It's really a matter of making it difficult enough to deter the largest amount of people. Even then, a wall doesn't make a lot of economic sense if it can be defeated with something as low-tech as a ladder or shovel. Trump backed himself into a corner when he shot his mouth off about a wall during the election. Now it doesn't matter if it's a cockamamie idea.....he has to follow through with it or look weak.
You may note that Don-John has now widened the definition of wall to include a fence. Before it's over, we'll see a bit of new wall, a lot of fresh barbed wire, and new electronic surveillance. A metaphoric wall, but he'll tell us it's huge, bigger than China's Great Wall, and that Mexico paid for it. (If Mexico is gonna pay, why is he shutting down the government over wall funding?)