The hill that overlooks the city of Santiago de Compostela is the site selected for the new City of Culture. Dominique Perrault’s proposal buries the building underground, turning the landscape of the hillside into its main façade. The topography becomes the architecture and the ground a full-fledged element of the program. But far from making the place disappear, this project relies on the incrustation of a gigantic optical instrument to flood each underground level of the structure with sunlight. This is a hybrid instrument, between a Fresnel lens and a kaleidoscope. Through a skillfully designed set of mirrors, it also reflects to the outside images of what is going on inside, and inside reflects fragments of the natural surroundings and the cityscape. The entrance is discretely placed on the side of the hill with only the impressive glass parallelepiped “instrument” emerging. This prism resembles a lighthouse in the landscape, watching over the historic city and guiding pilgrims to safe harbor.