Yet it is worth noting that what is valid for the entire composition is even more valid for the individual organisms. It is evident that, in his Campo Marzio, Piranesi presents a virtual catalogue, a typological sample book of models based on an exception that very effectively gives the lie to the rule. For further verification of this, note to which the degree to which the structures of Hadrian's tomb, the Pantheon, or the Theatre of Marcellus--among the few major monumental works in Piranesi's plates having a basis in reality--are arbitrarily reduced to minor, almost unrecognizable incidents, even as they are inserted into a continuum of fragments that deprives them of any autonomy as well as of the very status of "monument." They are exceptions that do not confirm a rule, then, and that lack any hierarchical organization. All of which permits Piranesi to show, simultaneously, just how vast the fields of these exceptions can be, once a generic classical reference has been appropriated by an experimentation based on geometrical deformations having no limits. But this same exaltation of the fragment also permits him to demonstrate, conversely, the uselessness of this breathless pursuit of exceptional structures.
Note, for example, the insertion of the officinae machinarum militarium within the triangle formed by the three large piazzas joined at the Pons Fabianus. The central star, formed by the intersection of two equilateral triangles, appears to be rotated with respect to its natural lying position, so that its vertices, aligned on the cross-axis, terminate in the little side rooms flanking the large site: the whole organism seems to be a kind of clockwork mechanism, in which, however, there is an independence of the parts and a lack of interest in formal qualities.
Also structured like hermetic "machines" are the organisms of the Circus Agonalis and the group of the Templum Martis and the Gimnasium Neronis, which form a kind of enormous notched wheel having differentiated spokes; the group located at the center of the Cripta Balbi, based on the intersection of two ternary groups of circular spaces and of a central rotunda defined by several concentric orders of columns broken by trapezoidal rooms on the traverse axis; or, finally, the group dominated by the Bustum Caesaris Augusti, an imposing collection of regular and irregular geometric forms one grafted on to the other according to the law of opposition. (Attention is also called, in passing, to the appearance of two phallic-shaped planimetric organisms converging on the hexagonal atrium, which foreshadow, perhaps with no other intent than a pure ludus geometrico, the project of Ledoux's Oikema and some of Soane's typological notions.)
But it is in the Horti Luciliani that the mechanical architecture of Piranesi reaches an extreme level of abstraction. Here, a complex of structures in semicircles and in sectors of circles obeys the rule of gemmation, as they revolve around the Atrium Minervae: an astonishing mechanism, in which Piranesi achieves the maximum refinement of his geometric instruments.
Manfredo Tafuri, The Sphere and the Labyrinth - Avant-Gardes and Architecture from Piranesi to the 1970s (Cambridge: The MIT Press, 1987), pp. 35-6.
Tafuri could have said so much more about the horti Luciliani.
Murcia City Hall, Moneo and Pienza, perhaps?
The City Hall Annex as (Semperian) weaving (into) the urban fabric?
aside: Among other things, Minerva is the goddess of weaving, so what exactly is the Atrium Minerva doing within the garden of satire?