The Monastery of St. John in Arles, founded in 512, was the birthplace of strict enclosure and the earliest female monastic Rule in the West. This book looks beyond the walls and offers a radical challenge to the established view of cloistered nuns as passive. Through an interdisciplinary 'inside view,' drawing on evidence from texts to textiles, it reveals the sisters, especially Caesaria the Elder and Younger, as agents whose 'diligent experimentation' profoundly shaped the legislation and spirituality of the cloister. The sisters emerge not as confined figures, but as powerful leaders-mothers, teachers, scribes, and mystics-who secured the monastery's extraordinary legal and economic autonomy and interceded for the city of Arles. This is an essential study of female agency and the lived experience of Early Medieval women's history.