Humans can climb a wall, with a rope or a ladder.
Everybody knows this.
Even the Great Wall of China was never intended to keep people out.
It was intended to keep out....wait for it......the horses!
The Mongols were fearsome on horseback, but on foot they were no better than the Chinese soldiers.
Modern walls do the same thing.
They keep out horses and camels and Toyota pickups and carbombs.
And that is why we need them, so the invader will be on foot.