There is not a square inch on this planet it is not feasable to build on. We have the tech to deal with all terrain. We lack the will.
Now if there is a 100 foot vertical cliff on the border, thats a natural wall. Otherwise, build.
As Trump said (as best as I can remember):
“You know what’s hard. Building a 95 story building is HARD. Building a wall is EASY. I’m a builder, I build things.”