Systematic survey of 1,000 km2 in a mountainous zone of the Mixteca Alta in Oaxaca, Mexico, has produced detailed architectural data at a large number of pre-Hispanic settlements. One unusual architectural form, circular stone foundations, apparently dates to the Late Postclassic period. Comparisons to similar architectural forms described in the archaeological and ethnographic literature of Mesoamerica and in the Mixtec codices suggest that they may have been sweatbaths. Analysis of their regional distribution and site contexts leads to interesting conclusions about additional contexts for sweatbath ritual. The symbolic link between royal birth, marriage, and sweatbath ritual is clear in the Mixtec codices. An additional use may have been in rituals affirming community identity and marking community boundaries. We suggest a further connection to the use of sweatbaths in Mixtec toponyms and their association with sacred places on the Mixtec landscape, both of which reflected the importance of marriage alliance and female royal inheritance in the territorial strategies of Postclassic Mixtec kingdoms.