The Plays of Nicholas Breakspear
At the end of the play, the two braggarts reappeared on the stage together to reaffirm the "reality" of the illusion. Having asked each other how they fared, the impresario of the fictitious performance answered nonchalantly that he had not really shown anything but the audience getting up to leave "with their carriages and horses accompanied by a great number of lights and torches." Then, drawing the curtain, he displayed the scene he had just said he had shown to his audience, thus rendering complete the incredible reversal of reality and illusion to the confused amazement of the real spectators, who were now finding themselves ready to leave and caught in the enchanting act of feigning the feigned spectators.
The Basilica Neptuni seems to be a building that Piranesi 'played' with within the Ichnographia. According to Nash's Pictorial Dictionary, Palladio drew a plan of the Basilica Neptuni (as illustrated therein), and Piranesi obviously knew this plan because he used it [and labeled it Xystus] within the Ichnographia. In my experience, whenever Piranesi makes what seems to be an obvious mistake within the Ichnographia, that's usually a sign that Piranesi is playing a(n inversion) game. That may not be the case here, but you should at least be aware of the (intentional?) transposition of buildings.
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