The Guild-seat of Trajan was excavated in 1938. It was built during the reign of Antoninus Pius, around the middle of the second century AD (opus latericium). An obtuse angle in the plan was caused by pre-existing Trajanic buildings.

The seat was built on the spot of two older domus, the remains of one of which (IV,V,16) were further investigated by the University of Geneva in the late 1990's. This house, below the east part of the guild-seat, was built in the first half of the first century BC and rebuilt after a fire, in the Augustan period.

The building was entered from the southern stretch of the Decumanus, through a passage more than 12 metres wide. In the passage were four marble columns, behind which is a semicircular exedra (A), 8 metres wide and 3.50 deep. In the back wall are two huge semicircular fountain-niches. The exedra is flanked by shops and a staircase. From the exedra vestibule C (7.50 x 13.50) was reached. A passage with two marble columns leads to a very large peristylium. The vestibule is flanked by almost identical rooms, dominated by rooms D and E, with a rectangular front part separated by two columns from a semicircular exedra in the back part. The exedrae open onto the rectangular rooms D4 and E4. To the north of D is a staircase, to the north of E a cistern. Rooms N and O had heated floors. Staircases F and K lead to cellars, staircases G and H to the first floor.

The peristylium (19.50 x 36 metres) orginally had a porticus on all four sides, with brick columns covered with plaster, and with marble bases and capitals. On the longitudinal axis is a long, narrow basin with many semicircular niches.

In the years 330-340 AD rooms were erected in the south part of the building (opus vittatum), and part of the peristylium now disappeared. Click here to open a detailed plan of the back part of the building in a separate window (Heres 1982, fig. 93). A passage with two columns with spiral decoration leads to the main hall P (c. 8 x 10 metres). In the back wall is a semicircular niche, c. 1 metre deep. From the pattern of the mosaic floor can be deduced, that this was a dining room. Around the hall are smaller rooms and a staircase, below which is a small semicircular wall-niche. Near the staircase, set against the outer wall, is a latrine with four seats. Four rooms were set against the central part of this outer wall, but their relation to the building is not clear.

The floors in the front part of the building are of opus sectile, while the walls are covered with marble and plaster. In the rooms in the back part of the building are black-and-white mosaics with geometric motifs, animals and erotes. On the walls are marble and plaster with painted imitations of marble. A large statue of Trajan, that gave the building its name, was found in the cellar below room L. A plaster cast is today in room E4. A small head of Minerva was found to the west of the vestibule. A statuette of Venus, two torsos of naked youths, a statue of wrestlers, and an altar with depictions of Anubis and Harpocrates were found in the building. A statue of Fortuna probably stood in the niche in room P.

The architecture (for example the size of the peristylium) suggests that this was a guild-seat, although the monumental entrance is atypical. The hypothesis is strengthened by the presence of a large statue of an Emperor, that is not to be expected in a private dwelling. A large and important guild owned the building, perhaps the fabri navales (ship-carpenters), who had a temple across the street (III,II,1-2). It is also possible that the premises were used by the navicularii Ostienses, mentioned in a fragment of an old inscription (not later than the period of Augustus) that was found in the building (AE 1955, nr. 178):

"To Pacceius, son of Lucius, quaestor with praetorian authority, the shipowners of Ostia, because he as the first a [sculptured picture?] ..." (translation Hermansen; the navicularii Ostienses are not documented otherwise). On another fragment of an inscription is a list of names, probably of members of a guild (album). A further inscription is a dedication to L. Volusius Maecianus, who taught law to Marcus Aurelius.