With this image, the first volume of Piranesi’s Antichità Romane moves to its final grouping of plates, each of which depicts floor plans or maps. Following his elaborate, multi-panel Map of Rome’s Aqueducts, the “Pianta della forma del Castro di Tiberio” consists of six elements: a hatched background that shows cardinal directions; on top of it, a piece of paper with the plan at the top and a measurement scale in the upturned curl at the bottom; two additional curling sheets of paper showing the perimeter of the soldier’s quarters and its numerical key; and, along the bottom of the page, the alphabetic key that identifies elements of the barracks and the wall. The structure is the Castra Praetoria, or the barracks of the Imperial Roman Guard (21-23 CE), whose extant remains were later incorporated into the Aurelian Walls. The first alphabetic annotation states that the plan represents the structure “innanzi al suo disfacimento” (A). From the material and textual evidence that remains, and from the combinations of evidence and artistry that characterize many of his works, Piranesi constructs this conjectural plan.  

His evidence, detailed in the first annotation, includes the structure’s ruins and façade, its depiction in both a bas-relief on the Arch of Constantine and medals, and works of ancient writers. The plan’s scale, placed precisely in the narrow space of the curling scroll, conveys the relative objectivity of units of measure (“Palmi Romani,” literally, Roman hands), but it also suggests, in its placement within a visual illusion, the other illusions that are inherent in representational media. Trompe l’œil effects that simulate the material surfaces of stone, paper, or both appear throughout this group of plans. In some, Piranesi distinguishes between extant and conjectural remains with different levels of hatching, with extant remains depicted with heavier inking and lost elements shown with lighter lines. 
The illusion in this image (close-up above) emphasizes Piranesi’s frequent play between the poles of evidence and invention. While measurement scales convey objectivity, the visual illusions of trompe l’œil mark art as art (Lolla). Such presentations of scales are common across Piranesi’s works, as in his “Cross-Section of the Mausoleum of Augustus.” This presentation of scale within the space of illusion, because it appears in a historical reconstruction based on various material and textual evidence, additionally calls attention to the creativity and artfulness entailed in Piranesi's archaeological speculation. (JB)