Dry stone construction is one of the earliest building methods. It was first used around 4000 BC. It is most often used to hold up soil but can also be used to construct buildings and bridges. The Incas used the dry stone technique to build Machu Picchu. Europeans built dry stone bridges in the Middle Ages. Making a dry stone construction is much likely working a puzzle. The stones are fit together, one next to the other. Dry stone construction doesn't use cement or steel. It relies on gravity and friction. Dry stone walls are very sturdy. What's more, they're eco-friendly.