The monastery of San Andrés de Arroyo (established in 1181) is one the best examples of Cistercian architecture in the Spanish region of Castilla y León. In this study, strontium isotope ratios were used to study the population of the recently excavated graveyard of this monastery. Twenty-nine individuals (of which 13 had preserved teeth) found in burials from the 13th and 14th centuries were subjected to analysis. Enamel ⁸⁷Sr/⁸⁶Sr ratio values obtained by using a multicollector ICP-MS instrument were used to distinguish between local and non-local individuals. Additionally, an anthropological study based on entheseal changes and other paleopathological conditions was carried out in all the exhumed individuals. This combined data allowed us to define the chemical and osteological diversity of a medieval Spanish monastery and its surrounding village. Among the usual profiles for a rural society, two individuals, both showing strong indications of being foreign to the area, were found to exhibit evidence of physical activities compatible with military training and activity. The present study can be considered as the first experimental indication that during the 13th and 14th centuries Spanish monasteries served as a last refuge for soldiers, which is compatible with historical records and previous indirect evidence.