Piranesi’s Campo Marzio converts antiquarian reconstruction into an architectural field whose evidentiary fragments and speculative completions remain productively entangled. The project begins from ruins, inscriptions, literary testimony and the uncertain limits of the ancient Campus Martius, but its great ichnographic ambition exceeds conventional documentation. Piranesi attempts to make the total field visible “at one glance” while acknowledging that much of the ancient fabric has disappeared or survives only as buried or illegible traces. The iconic idea is therefore not archaeological certainty but a reconstructed urban totality assembled from unequal evidence. Monument, conjecture, fragment and graphic invention coexist inside one dense planar system. Methodologically, the project treats the plan as an epistemic surface capable of holding incompatible degrees of certainty without reducing them to a single status. Its wider bridge is to speculative cartography, archival reconstruction and field-based urbanism, where knowledge emerges from the disciplined arrangement of incomplete traces.