The discovery of a fragmentary manuscript of medieval polyphony in Seville Cathedral sheds light on the cultivation of polyphonic music at the royal court of Alfonso X. This paper explores the coexistence and reciprocal influence of the vernacular Cantigas de Santa María and Ars Antiqua polyphony in the context of Seville Cathedral, one of the main stages of the royal ceremonies in Alfonsine Castile. The Chapel of the Kings, installed inside a consecrated mosque, materializes the fascination of the Castilian monarchy for both Gothic and Arabic art. This contrasting duality must have been experienced to the highest degree when the royal clerics and cathedral cantores performed Parisian polyphony against the backdrop of the mosque’s arches and arabesques.