Compartments numbered 1 to 15 with key: 1 Ingresso principale nel Tempio. / 2 Portici all'intorno coperti, con colonne. / 3 Stanze ad uso de Bagni vedendosi tanto le pareti, / che il pavimento tutti lastricati di marmi / ed avanti un rivo alla porta alto due Palmi in / circa che tant'altezza d'acqua entrava in quelle / camere. / 4 Ingressi laterali. / 5 Camere esterne, che' per mezzo d'una Scala ognuno / saliva in Camere superiore. 6 Scale maggiori che portavano al di sopra / 7 Piedestalli con Statue / 8 Canale d'Acqua per le scale / 9 due Anelli di mettallo, che anche al'presente / esistono dove di legavano le Vittime. / 10 Tempio rotondo in mezzo un'Ara. / 11 Quatro Scale per ascender al medesmo. / 12 Vasi dell'altezza 3 1/2 piedi ove si metta la acqua / per lavarsi dopo fatti li sagrificj / 13 Piedestalli con Statue. / 14 Collonna maggiore ove si passava alla Cella o ristretto del tempio / 15 Bagni magnifici laterali
In his Italian notebook with sketches, 'Italian Sketches and Mema', 1778-9 (SM volume 164, 15) Soane wrote: Went to the Temple of Jupiter Serapis; this was on 1 January 1779. In his Italian sketchbook with notes, (see in Sketchbooks catalogue: Italian Sketches', 1779 - SM volume 39, 9v,10r) Soane made some rough drawings of details of what was, in fact, not a temple but a city market. The building was excavated from about 1750 and was then 'constantly in a state of semi-submersion under water or debris' (du Prey, op.cit, 1977, p.166) and thus difficult to measure. The reconstruction plan is pricked for transfer and was presumably copied from another architect. The Italian inscriptions are in Soane's hand; the lack of a scale bar and labelled dimensions also point to this being a copy.
A translation of the labels to this plan (contributed by Stephen Parkin) is as follows: 1 Main entrance into Temple 2 Covered colonnade 3 Rooms for the use of bathing with the entire walls and floor covered in marble and with a channel about two palmi deep at the doorway since this was the depth of the water which flowed into the room 4. Side entrances 5 Outside rooms from where stairways led to upper rooms 6 Large staircase leading to the upper floor 7 Plinths with statues 8 Water channel along the stairs 9 Two surviving metal rings to which victims were bound 10 Round temple with an altar at the centre 11 Four steps to climb to the same (that is, the round temple just indicated) 12 Three and a half foot tall vases which were filled with water for washing after the sacrifices had been performed 13 Plinths with statues 14 Large colonnade through which there is entry to the cella or enclosed area of the temple 15 Magnificent baths at the sides of the building
When excavations began, the discovery of a statue of the Egyptian god Serapis led to the assumption that the building was a temple. The plan with its continuous rows of shops on all sides indicates nothing more than a market though a 'cella' fronted by four tall columns and a circular structure in the centre of the colonnaded courtyard must have suggested another function.