I think they made the stones using molds. However, I believe that the molds had a protrusion on the bottom and a corresponding cavity on the top to fit this protrusion. This way, they could stack them on top of each other, making people think the wall was made from a single piece. Moreover, when the stones were fitted together, they could pour the liquefied stone into the cavity to act like cement, ensuring the stones wouldn’t collapse, and then let it cool.