Julius Caesar and his engineers came up with an ingenious series of defenses during his siege of the city of Alesia. They built palisades, towers, ditches, moats, trenches filled with caltrops and barbs, and berms embedded with the sharpened boughs of felled trees facing out.
If we get a wall, we get a wall. But I think things like Caesar’s works placed strategically along the border would be as effective. The British built a natural “hedge line” across India in the 1800’s that turned out to be an effective barrier. Something of the sort could also be employed.
Wikipedia has an article called The Great Hedge of India. It was a barrier for collecting taxes. The Chinese also had a man-made natural barrier called The Willow Palisade.