Hey, check out these topographical maps of the Rocky Mountains, where they cross the border into Mexico. It's gonna take a bit of grading to put a wall across those.
Why in the world world we need to build walls over the Rocky Mountains? Wouldn't they be barrier enough? Just block the passes and put some Dwarves and Elves with those new pepper ball launchers atop those small barriers.
Heck, that'll make it even easier than I thought.
