The Casino with Semicircular Arcades. Axial view from west to east: in the foreground is the Casino, in the background is the Winter Palace (Edificio con Peschiera).

Starting again from the Pecile (n. 16), the great square linking different quarters of the Villa, one meets the access to a group of buildings identified as the Imperial Complex (Residenza Imperiale) (nn. 20-24) within the Villa itself, because of their monumental appearance, the magnificent wall and pavement decoration using precious marbles, and because its buildings reproduce the basic elements of a roman house or domus. (For a discussion on this subject, see De Franceschini 1991, La Residenza Imperiale, p. 541-546).

The monumental access can be identified in the the Casino of Semicircular Arcades (Edificio con Tre Esedre) (n. 20), which was decorated by a rectangular fountain, a quotation of the atrium and impluvium of the ancient roman domus. This building had a central squared porch on which opened three semicircular courts decorated by fountains, and surrounded by semicircular porticoes. The walls of its main hall still bear the traces of great marble reliefs. From the Casino of Semicircular Arcades it is possible to see-through in a wide perspective up to the Winter Palace (Edificio con Peschiera) (n. 22) (see picture). Here as elsewhere there was no direct access from one building to the other, but a concealed and meandering way, passing throug side rooms, thus creating a security check-point.

The Garden Stadium seen from above. In the background is the Nimphaeum with semicircular water staircases.

In between the two cited buildings there was a garden decorated by fountains and an open-air Triclinium, the Garden-Stadium (Ninfeo Stadio) (n. 21). It had three courts, the central one linking together the Casino of Semicircular Arcades (n. 20) and Winter Palace (n. 22), through a double porch. The second court was on its northern side, with three halls and a garden decorated by water basins. The central hall had an apse, the other in the north-eastern corner still has some frescoes and opus sectile pavement; inside, small door lead to a service corridor linking the Garden Stadium to the Pecile (n. 16), the Philosopher's Hall (n. 17) and the Terme con Heliocaminus (n. 19).
The third court had a nymphaeum with semicircular water staircases at its southern end, similar to the one seen in the Imperial Palace (n. 12). Before the nymphaeum is an open colonnade supporting the roof of a summer triclinium, flanked by channels for running water. Its shape recalls that of the Auditorium of Mecenate in Rome or the Voliera of Varro. Similar gardens

The great basin or Natatio of the Winter Palace (Edificio con Peschiera). In the niches were probably statues, stolen in late antiquity.

are featured in the so-called Stadium in the imperial Palatine Palaces of Rome, or in the Villa of Domitian in Castelgandolfo near Rome. These were not real 'Stadia', but internal gardens with the shape of a Stadium, belonging to the architectural iconography of imperial palaces. (On this subject see De Franceschini 1991 p. 510). The Garden Stadium was studied and published in an excellent book by Hoffman (see Hoffmann 1980).

One of the four corridors of the Cryptoporticus underneath the Winter Palace (Edificio con Peschiera). It secured a fresh strolling place during summertime, and it was lighted by openings in its vaults.

On the eastern side of the Garden-Stadium is visible the imposing height of the Winter Palace (Edificio con Peschiera) (n. 22), which was provided of a heating system for winter-time: this, together with its precious decoration, points out that it was reserved to the Emperor. The building linked different levels, the lower one corresponding to the entrance and the Pecile, the higher one corresponding to the Palazzo Imperiale and surrounding grounds. To do this, it was necessary to build three different floors: the lower one had a series of rooms overlooking the Garden-Stadium, linked by a long corridor.

The Firemen's headquarter (Caserma dei Vigili) one of the best preserved service buildings of the Villa. Similar to the one in Ostia, had wooden staircases to reach the upper floors.

A large staircase lead up to the intermediate floor, which served as podium or basis villae for the upper floor. It consisted of a spectacular hidden Cryptoporticus, on the vaults of which are still visible the signatures of ancient visitors, as Piranesi or Quarenghi (and, unfortunately, of modern ones). The upper floor, finally, had vast halls overlooking the beautyful panorama, from Roccabruna to Tivoli.
The Winter palace was entirely paved in opus sectile made of precious marbles; its walls still have the traces of a marble revetment reaching up to the ceiling. On its eastern side, resting upon the underlying Cryptoporticus, is an internal porch decorated by a water basin - the Peschiera - with niches for statues. Exit doors lead to a series of paths going to the Imperial Palace (n. 12) or Piazza d'Oro (n. 15).

Not far away is the Firemen Headquarter (Caserma dei Vigili) (n. 22a), a squared secondary building paved with bricks, which was conveniently located in a central position. Quite well preserved, had an internal court and galleries accessible with wooden stairs. It also featured a multiple latrina - used only in secundary or servile buildings. Here lived servants and slaves and, possibly, firemen, since the danger of fires was very high at the time. The building is quite similar to the Caserma dei Vigili in Ostia.

The Imperial Residence complex had a monumental entrance hall - the Casino of Semicircular Arcades (n. 20), an internal garden with summer Triclinium – the Garden Stadium (n. 21), and the main Palace with winter heating – the Winter palace (n. 22). To be complete, it needed a thermal plant, which has been identified in the Small Baths (n. 24) linked to the other buildings by the Quadriporticus (n. 23) (see De Franceschini 1991 p. 541-546).

The Quadriporticus, linking the Small Baths with the rest of the Imperial Residence. It is decorated by a Nymphaeum inherited from the preexisting republican Villa.

The Octagonal Hall of the Small Baths (Piccole Terme) is one of the masterpieces of hadrianic architecture.

The Quadriporticus (n. 23) was built in the corner between the Casino of Semicircular Arcades and the Garden Stadium, and was a square courtyard surorunded by porticoes. Its southern side was decorated by an ancient republican nymphaeum, with curved walls and niches, inherited from the previous Villa. A small door – one of the many concealed access ways seen in the Villa - entered into the Small Baths (Piccole Terme) (n. 24), which were the Emperor's baths, as shown by their architetctural complexity and precious decoration.
The Small Baths (n. 24) are one of the most complex and spectacular buildings of the Villa, conceived as a labyrinth with diverging axial perspectives and long suites of rooms. They are a sample of multi-shaped domes, the most famous of which is the Octagonal hall, one of the masterpieces of roman architecture: its eight sides were alternatively curved or straight, and supported a concrete circular dome with an opening (oculus) at its center. On the eastern side of the building was an open air rectangular court (a Palestra), from which a door entered in the oval frigidarium, featuring two apsed basins for cold water. There also was a circular hall intensely heated, and an oval pool for hot water. The decoration includes some of the most elegant opus sectile marble pavements of the Villa. The walls stille bear traces of marble revetment, reaching upo to the ceiling. The Small Baths were never studied in detail, nothwithstanding their architectural excellence.

The Great Baths, paved with simple black and white mosaic, were meant for the servant's use.

Near the Small Baths were located the Great Baths (Grandi Terme) (n. 26). Some scholars believe that they were meant for men while the Small Baths were for women, but this is not true. The difference between Small and Great Baths is pointed out by the quality of their decoration, which shows that its users belonged to a different social rank, not sex. The Small Baths had precious opus sectile pavements, were meant for the Emperor's use and belonged to the noble quarters. The Great Baths had simple black and white mosaic, as the Hospitalia, and were used by servants and slaves (it seems that they were never finished to be built). A system of internal paths and accesses completely isolated the Great Baths from the surrounding buildings: people coming from the Small Baths or from the Winter Palace could go to the Praetorium Pavilion (n. 27) using a Cryptoporticus and by-passing the Great Baths. The Small Baths, instead, were directly connected to the Casino with Semicircular Arcades (n. 20) and also with the upper level of the Villa by a staircase, reaching the Winter palace (n. 22) and from there also the Praetorium Pavilion (Padiglione del Pretorio) (n. 27).
The Vestibulum (n. 25) gave access to different quarters of the Villa. On its northern side was a monumental staircase, the main entrance of the villa with the paved road in the shape of a rectangular ring described in the section Pecile and Hundred Chambers. Inside there were several courts with porticoes, surrounded by cubicula. On the northern side there was a path leading to the Pecile. The eastern part of the Vestibulum, instead, gave access to the Small Baths and to the nearby Cryptoporticus, from which staircases gave further access to the Winter Palace or the Praetorium Pavilion. Finally, a third exit on the southern side gave way to the path going to the Canopus. The Vestibulum is paved with opus sectile or mosaic. For long years its eastern part was completely hidden by a small wood; in recent times it has been cleaned and restored.

The Praetorium substructures supported the side of the hill east of the Canopus. On their upper floor there was a panoramic Pavilion overlooking the whole Villa.

The Praetorium Pavilion (n. 27), from which one could see the surrounding panoramic view, rested on a series of substructures very similar to those of the Cento Camerelle, with high vaulted rooms containing the side of the hill. The Pavilion was paved with opus sectile and bears traces of wall marble revetment; il belonged to the imperial quarters and was similar to the Tempe Pavilion. The vast area behind it (east and south) has never been explored; Canina believed that in this part of the Villa was an Hippodrome.

West of the Praetorium, a beautyful path leads to one of the most famous and photographed buildings of the Villa, the Canopus (n. 28). It has been excavated by the Jesuits in the XVIII century, finding sculptures of egyptian subject, and later on in 1950 by Aurigemma, who discovered many statues among which the copies of the Cariatyds of the Herecteion of Athens. Unfortunately his excavations did not consider stratigraphy, and its many articles and books just deal with the sculptures that were found.

The Canopus, with its long basin decorated by statues, columns and pergulae, is one of the most famous and celebrated buildings of Villa Hadriana.

The Canopus consisted of a long water basin flanked by pergulae, which lead to a monumental building in the form of a shell-shaped grotto, decorated by niches, fountains and waterworks. Its walls originally had marble revetment, while the vault was decorated with mosaic. At its center was a semicircular stibadium, or triclinar bed for the guests: the Canopus, in fact, was a spectacular summertime open-air dining room. In the center of the dome there is a sort of grotto, entering inside the hill, decorated by niches for statues and waterworks. Water gathered in an undelying basin and then overflowed, flowing into a series of small channels surrounding the triclinar bed. In the back of the building were several other rooms paved with opus sectile, and two small single latrinae. On the western side, a long room probably used as a kitchen was originally decorated by frescoes, which were detached in the 1960's. Two staircases lead up to the hill behind the Canopus, where has been found a water basin and a small aqueduct supplying the waterworks. In this area was also identified a Nymphaeum.

The great shell-shaped half-dome of the Canopus, a spectacular open-air summer banquet hall, decorated by niches with statues and fountains.

The half dome of the Canopus was an artificial, 'virtual' grotto, and since in its basin was found a fragmentary sculpture representing Scylla, it probably recalled other imperial triclinia such as the one in the Grotto of Sperlonga, in the villa of Tiberius, where was found a marble sculpture representing Ulysses blinding Polyphemus. In the hellenistic royal palaces, the grotto was part of the decoration and iconography; in roman times the grotto was very popular, even in small villas and private homes, as seen in Pompeii and other archaeological sites. In the imperial villas the grotto was a 'must', and reached monumental size, as in this case.
It is important to remind that Aurigemma's reconstruction of the columnade surrounding the Euripus basin of the Canopus is hypotetical, and was not supported by real excavation evidence.