Photo: Ihiroalfonso, CC BY-SA 3.0 via Wikimedia Commons
The unusual alignment and architecture indicate that this was an astronomical observatory.
© OpenStreetMap contributors
Plaza de los Danzantes
Replicas of a group of carved stones found near Building L, noted for fascinating Olmec carvings of naked men in various contorted positions. The original archaeologists described them as "dancers", but a more recent interpretation is that these are mutilated victims about to be sacrificed.
Plataforma Sur
A very large pyramid with an open plaza area at the top. This is one of the best viewpoints of the ancient city.
Juego de Pelota Grande
The Zapotec used a very different layout from the Mayan ball courts at sites like Coba or Chichen Itza.
Plataforma Norte
Large, high platform with several structures (possibly temples) at the top. If you're in a hurry, you could go straight to the north platform and back in 15 minutes for a great view of the site.
The artwork on the back side of the M$20 banknote depicts the view from atop this small unexcavated ruin.
Monte Albán Site Museum
This small museum has some excellent interpretive displays plus a number of relics excavated from the site in the early 20th century. The highlights are its collection of stelae and the original Danzantes carvings. All the signs are only in Spanish. There are also restrooms, a small bookstore and a cafe.