On the north side of the portico, almost in the centre, there is an ancient Nymphaeum also built in opus incertum.
Inside it had niches for statues, from which jets of water flowed.
On the sides of the Nymphaeum the Greek Library (to the west) and the Latin Library (to the east) were built.
The Courtyard of the Libraries was a closed garden that served to give access to several nearby buildings.
On the north side two narrow corridors on either side of the Nymphaeum led to the Upper Terrace of the Libraries, and some doors open towards the Latin Library and the Greek Library.
On the east side there were two other entrances towards the Hospitalia.
On the south side there is the entrance to the Cryptoporticus with Mosaic Vault. Two symmetrical staircases, built inside the retaining wall, went up to the Imperial Palace, together with a third staircase which is still in use today.
Finally, on the west side there is an access to the Baths with Heliocaminus and a staircase that descends to the Maritime Theater, which is on a lower level.
The opus sectile floors prove that the building belonged to the noble quarters of the Villa reserved for the emperor.