The Hypostyle Fountain is the largest and perhaps the earliest monumental fountain in the ancient sanctuary of Labraunda in Caria (Turkey). Built wholly out of massive gneiss blocks, this impressive columnar structure, along with two marble monumental gates, served to articulate access to the entire sanctuary for pilgrims from around the region. Today it provides important new evidence for vernacular traditions of monumental architecture in Hellenistic Anatolia. In terms of design and facture, the entire fountain— and particularly its unconventional roof—is more akin to buildings such as the remote temple at Gerga or the fountains in the mountain fortress of Theangela than to the famous Mausoleum, which Labraunda’s best known patron, Mausolus, built in Halikarnassos. Our paper will present the preliminary results of two years of detailed architectural study, including a stone-by-stone analysis of the roof that has shed light on the actual process of design and erection of the building. Of special interest to scholars of ancient architecture are our findings regarding the workmen’s chaine operatoire and the numerous ad hoc solutions to problems that arose during construction.