Imagine the folks who built the Roman Coliseum, and all those beautiful medieval cathedrals and European castles with their defensive walls. How did they get all that stone way up to the top rows? For the larger stones, it would have taken a whole team of men to carry each one to those heights. But that wasn't necessary, because they had machines, powered by humans. Guédelon Castle in France has been under construction for at more than 20 years now. It's not that construction has halted or funding has dried up, but they are using 13th-century technology to build it. That includes a treadwheel crane, also called a treadmill crane (because we know treadmills better than we remember grain mills). Using a large wheel with a person walking inside, they transferred the distance walked into lifting power through pulleys. Tom Scott demonstrates the treadmill crane at the Guédelon Castle construction site.