He explained, “One of the things with the wall is you need transparency, you have to be able to see through it. In other words, if you can’t see through that wall – so it could be a steel wall with openings, but you have to have openings because you have to see what’s on the other side of the wall.”

In another instance, he said the need for transparent walls was from the possibility of harm from projectiles from the southern side. Trump explained, “as horrible as it sounds, when they throw the large sacks of drugs over, and if you have people on the other side of the wall, you don’t see them–they hit you on the head with 60 pounds of the stuff? It’s over.”

He reiterated the need for a see-through wall during a subsequent press conference, where he noted that it would be “a more powerful wall and a more beautiful wall.”

Business Insider – Trump says his wall along the US-Mexico border will have to be see-through so people don’t get hit with giant sacks of drugs
Business Insider – Trump doubles down on the need for a ‘see-through’ border wall because he wants ‘to be able to see’
USA Today – ‘A WALL is a WALL!’ Trump declares. But his definition has shifted a lot over time
Wired – Let’s Talk About Trump, Border Walls, and Flying Heroin