There is an article that was posted recently https://www.earthasweknowit.com/pages/inca_construction (HN discussion https://news.ycombinator.com/item?id=46342950 | 172 points | 55 days ago | 46 comments) The most important bit is in the middle https://www.earthasweknowit.com/photo/peru_cusco_masonry_con... The stones were concave, so the match perfectly only outside and have some kind of mortar inside the wall.