Not only do we not know who built these ancient fortress walls, but we also don't know how they were built. They date back to pre-Incan times in Peru, and they're so perfectly constructed that a sheet of paper couldn't pass between the cracks.
Some of the stones that make up the walls weigh more than 200 tons. It would take a large team of people with advanced machinery to build something like this today, but the Incans had none. How did they even lift the stones?