Monte Alban is a plateau holding the ruins of a (mostly) Zapotec city. It's just outside the modern city of Oaxaca, and by just outside I mean we could even see it from our hotel room. Those two lumps on the left side of the plateau are part of the temple complex.
The ruins are well-preserved, at least for the central part of the city, and very impressive. It's a couple hours to do a basic tour of the buildings and the small but interesting on-site museum. It's fairly easy to get to it from Oaxaca - for one you could just take a taxi. We took a bus from another Oaxacan hotel which was simple enough. The hardest part is finding the correct bus again in the chaotic parking lot complex when you're ready to return.
These 'I' shaped ball courts are ubiquitous across Mexican ruins. This isn't substantially different from some of the Mayan sites we've visited. They don't entirely know how the game worked or why the court had this shape.
The platforms at either end of the city (north and south) can be climbed and they both provide spectacular views of Monte Alban and pretty nice overviews of Oaxaca and its valleys too. The general layout is obvious in this picture. Most of the ceremonial buildings were set around the perimeter of the plateau with a few buildings down that center stripe.
The closest building in that center set is the 'observatory'. It is the only building set at an angle to all the rest and it also has an unusual arrow shape. The general theory is that it was aligned astronomically. There are carvings around the base of it as well. Current theory on these is that it was a list of all the other cities that owed fealty to Monte Alban.
One thing worth noting here is that there are very few shade trees available. It's generally hot and dry and mostly exposed to full sun. Bring water.
One of the most impressive sites are the 'danzantes' or dancers. These stones were located in one of the buildings and some of them are still in situ there (you can crawl in and see them there). Most of them are arranged in a line next to the building though. There are a lot of theories on this too depending on your source but in general it involves prisoners of war and mutilation.
The North platform is a complex set of structures. Once on top of it there are further pyramids and an unusual site in meso-American ruins: a colonnade, or at least the remnants of one.
The north platform also has the best overall view of the site. It's hard to tell from this angle but there is another tier in the distance just past the large tree. Monte Alban was occupied roughly from 500 BCE to 1000 CE. As often seems to be the case, it's unknown exactly why the city was abandoned.