This impressive space, located between the Imperial Palace and the Guard Barracks, had an uncertain function. Three parts may be distinguished: on the west, a curved, open-air courtyard (1) with plantings in the center and three statue niches on an apsidal wall (one of which displayed the statue of a faun); next, a series of five rooms (4-8); and, on the east, a large space (9) recalling a basilica with side aisles delimited by pillars connected by an architrave of the Doric order (hence the name of the structure). It is probable that the basilical space was roofed. The side aisles (10) were covered by barrel vaults; the side walls and floors were in opus sectile. In view of its design, décor, and proximity to the Imperial Palace and Piazza d'Oro, the hall may well have been used for imperial meetings and audiences. The structure dates to Phase I (118-125 CE).