This presentation examines two ruined monumental architectural complexes in prehispanic Oaxaca, Mexico: The Main Plaza of the mountaintop city of Monte Albán in the Valley of Oaxaca and the acropolis of Río Viejo on the coastal floodplain of the Lower Río Verde. Both complexes were built during the Formative period and became the ceremonial centers of important cities and foci of politico-religious life for larger regions. The two complexes, however, had vastly different histories from their initial construction through their time as ceremonial centers and their afterlives as decaying ruins. Although both fell to ruin during the prehispanic era, they continued to be places of intensive affect that were central to the constitution and transformation of broader communities in their respective regions. Drawing on new materialist approaches as well as on Native American ontological perspectives in Oaxaca and elsewhere in Mesoamerica, I argue that even as ruins, both complexes were powerful animate beings linked to agricultural fertility, sacrifice, ancestors, and cycles of creation. The presentation considers how the material vibrancy of these ruins differed in ways that both brought together and destabilized community and hierarchy. After the abandonment of Monte Albán, its Main Plaza, viewed from afar by the people in the valley below, continued to assemble substances important to human well-being including rain, earth, mountain, sky, ancestors, and divinities. People from communities in the valley periodically journeyed to the plaza to make sacrificial offerings thereby constituting a broader identity and community, although one that was much changed relative to the time when Monte Albán was a city. In contrast, the earthen architecture of the acropolis, located in the center of Río Viejo, rapidly eroded and decayed in the tropical lowland climate. The Late Classic period reemergence of hierarchy in the lower Río Verde Valley activated material memories of rupture held in the ruins that threatened and resisted new forms of community and political authority. As a result, nobles were drawn back to the architectural complex to appropriate that power though the emplacement of a complex offering that may have healed and reanimated the building. The processes of ruination at the two monumental complexes discussed here therefore actualized different capacities contributing to the gathering of a new kind of community in one case, and its resistance and eventual dissolution in the other. This presentation makes the broader point that even ruined buildings can be powerful in ways that threaten, resist, empower, or transform human projects.