The fourteen plates of the Carceri, described on their title page as "capricious inventions," are probably Piranesi's best-known series. These structures, their immensity emphasized by the low viewpoint and the small size of the figures, derive from stage prisons rather than real ones—Piranesi had created an earlier prison fantasy (in his Prima Parte) that is closely based on stage designs by Ferdinando Bibiena and Filippo Juvarra. Spatial anomalies and ambiguities abound in all the images of the series; they were not meant to be logical but to express the vastness and strength that Piranesi experienced in contemplating Roman architecture, to which he remained in thrall throughout his life. While elaborate theories have been developed to account for this series, it has also been plausibly suggested that Piranesi chose an architectural subject devoid of ornament and requiring little detail or textural differentiation, so that he could isolate the issues of perspective and spatial structure. In this series of variations on a theme, Piranesi attacked his copperplates with a boldness and spontaneity unmatched in any other work of his time.