Porticus Gratiani Valentiniani et Theodosius near the Arch of Gratian, Valentinian, and Theodosius
Vincenzo Fasolo, "The Campo Marzio of G. B. Piranesi".
I forgot to mention this before--the Castle Howard-Ichnographia Campi Martii connection may also be found in some of Piranesi's individual plans within the large plan. For example, the Porticus Gratiani, Valentiniani et Theodosii somehow reminds me of the plan of Castle Howard--the plans are not what you would call 'alike', but they both strive for maximum effect through minimum means.*
*What is ingenious about Vanbrugh's ground plan at Castle Howard was the way that it contrived to create the maximum visual effects of architectural scale and magnificence out of the minimum number of rooms. Both externally and internally it looked and felt like a much larger house than it actually was, if calculated in floor space.
--Charles Saumarez Smith, The Building of Castle Howard (1990), p. 55.