In the middle of the main plaza is a row of buildings, which included a series of temple platforms (Building G, H, and I) and the unusual Building J, probably the most famous construction of Monte Albán. The building was constructed in the Terminal Formative period (ca 100 BC - AD 200) and has an unusual arrowhead outline.
Its orientation, toward the south-east, instead of north-south as is true for the rest of Monte Alban buildings, made archaeologists suggest that Building J was used for astronomical observations, and therefore it has been dubbed "The Observatory".
Building J's external walls are covered by more than 40 carved stones called “conquest slabs”, which listed the place name of communities that were supposedly conquered or colonized by Monte Alban.