This building (or rather complex of buildings) was situated on the narrow stretch of land between the basins of Claudius and Trajan, to the north-west of the latter basin. The early investigators (Labacco, 16th century) called it Palazzo delle Cento Colonne; later it was called Palazzo Imperiale. It may indeed have been an Imperial residence, that was normally used by the procurator of Portus, the supervisor of the harbour district. "Excavations" were carried out by prince Alessandro Torlonia in 1864-1867. In this period Lanciani visited the site. Because the ruins were quickly covered with earth again, he could only make a partial plan. The various descriptions of the site are confusing and there are many uncertainties. It seems that the complex was behind the upper part of the Terrace of Trajan (12-13).
At least part of the complex can be dated to the reign of Claudius: lead waterpipes with the name of Messalina, Claudius' wife, were found here. It was rebuilt at the end of the reign of Trajan or the beginning of the reign of Hadrian (opus mixtum of excellent quality). Walls of the complex coincide with the angles of Trajan's basin. Inside two fragments were found of inscriptions mentioning Trajan, dated to 112 AD. Further work took place in the Antonine period. Brick stamps were found from the years 114, 115, 116, 123, 144, 154 and c. 157 AD.
According to Lanciani the west and north facade of the complex consisted of a very long porticus with many columns. A room behind the south end of the porticus ended in a large apse. It had a mosaic floor with a depiction of a Centauromachy. A bit further to the north was a large hall with on all four sides large apses with niches. It contained marble columns. Below the marble floor was a heating system (hypocaust): apparently it formed part of baths. Again a bit further to the north was a round hall. Not far away was a large hall with niches, near which statues and busts were found of a Muse (h. 2.12), Aesculapius (two: one small, the other 2.23 high), an athlete, Septimius Severus, Leda, a philosopher, and a slave (Lanci, BullInst 1865).
Somewhere in the complex was a temple, in which - in 1794 - fragments were found of a statue of Hercules. Lanciani also reports the presence of a small theatre (see nr. 27A) and, at a distance of about 100 metres from it, at least thirty-five underground rooms and corridors.