Monte Alban is located on a mountain top on the western edge of Oaxaca city.  Of all the ruins I've seen, this is my favorite.  When I'm there, I can imagine the color and activity that passed away nearly 500 years ago.  If you go, plan at least 4 hours, six is better. 

The first time I visited Monte Alban I counted five different styles of arches for doorways and passages.  I thought "Isn't that amazing how the architects changed their construction methods over time!"  On my second visit our guide explained that some of the openings were made by the archeologists as they explored the site. Now I don't know which arches are original and which are not but it does explain why I have so many pictures of them!