Chaco Canyon is a very old city that was built by the Anastazi which were the early people who lived in a major part of the four corners area of the United States. The stone masonary work is amazing and indicates hundreds of thousands of man hours to construct in an area that probably at the time had less than 16 inches of rain fall annually. (remember that you have to water and feed the people in the city also and their families).
Now don't mistake the more sloppy work along the tops of the walls-that is repair work done to stabalize the structures-along with additional beams done by the National Parks Department (some done back in the 30's and 40's which has not been upgraded to the standards used today).
Some of these buildings rose to 4 floors in height and one of the groups had just one entry door to the whole complex from the outside-many of the doorways that you see on the perimeter were not orginally there but some of the windows were. So for some rooms to get into them you may have had to go through a dozen other rooms to reach them with no windows to light the way.
The allignment of many of the buildings is based on astronomy and there is even a road on the top of the Mesa that was built that ran for over 30 miles straight north for some unknown reason.