This thesis presents a new comprehensive study of the possible functions of a space discovered underneath the Duomo of Siena in the late 1990s. The hundreds of graffiti onto the walls of the site inspired this thesis and allowed a rethinking of the planning of the building, its famous frescoes, and their reception. The research was started by considering previous publications on this space, which mainly focused on its structure and painted cycle. The first chapter investigates the structure of the site within the construction phases of the Duomo and the urban organisation of the city, together with symbolic analysis. The second chapter offers new analyses of the painted cycle in terms of narrative strategies, reconstruction of the lost episodes, new iconographic identifications of some of the scenes, and message of the painted cycle. It then investigates the relationship between the paintings and liturgical uses of the space. The third chapter focuses on the graffiti by framing them in the wider context of this medieval phenomenon, as well as attempting the identification of the possible makers of these signs. A study of the meaning and intention behind these markings is provided. Finally, the thesis includes a catalogue of the graffiti that provides information on each item.