One of the things with the wall [on the Mexican border] 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.

Source

Trump's 30' tall, 2,000-mile wall on the Mexican border which was going to be paid-for by the Mexican government is now a 700-900 mile fence with solar panels to be paid-for by the US taxpayers!