The Republican Monument has been interpreted as a funerary monument of an important citizen, originally with an aedicula or altar on top. The funeral chamber must then be in or below the base. The monument is situated to the south of the Decumanus, opposite the theatre, between the east gate of the Castrum and the Roman Gate, which forms part of the city wall from the first century BC (probably the work of Marcus Tullius Cicero). Tombs were not allowed inside the city walls, so the monument has to be older than the wall from the first century BC.
In trenches in this part of town countless remains were found of buildings from the second and first century BC, but other tombs were not found. This is reminiscent of the situation outside the Porta Marina, where only two large tombs, both of leading Ostian citizens, have been found. In the second century AD buildings were erected to the east, south and west of the monument. It remained visible from the north, from the Decumanus.
The monument has been studied in detail by Laura Sole. What remains is a square base (6.10 x 6.10) of large tufa blocks, with a cornice at the bottom. Three layers have been preserved, but the excavator, Dante Vaglieri, saw traces of another five layers, which means that the base was at least 3.80 high. The monument is at a low level, 1.40 below the present level of the Decumanus and 0.70 above the sand. The following layers are documented between the Castrum and the Roman Gate:
- A raising of c. 0.70 on top of the sand. On this layer rest the Republican Monument and the Cippi of Caninius, that have been dated by Meiggs to c. 150-125 BC.
- A raising of c. 0.80 on top of layer 1. On this layer rests the Republican Temple on the corner of Via dei Molini. This raising should probably be dated to the reign of Claudius.
- A raising of c. 0.l5 on top of layer 2, probably from the reign of Domitian.
- A raising of c. 0.25-0.75 on top of layer 3, probably from the reign of Hadrian.
- A final, slight raising, probably Severan, taking the final level to c. 2.00-2.50 above the sand.
A comparison with similar monuments leads Sole to a date of the monument in the period 120/110 - 70/60 BC, and most likely the early first century BC. According to Filippo Coarelli the monument was a tomb from the late first century BC, of Gamala, son of the builder of the Quattro Tempietti to the west of the theatre. He suggests that there is a relation with the Warehouse of Hortensius to the east, and that this warehouse was built by Gamala. An objection to Coarelli's hypothesis is, that the funerary monument would then have been built inside the city wall, which is abnormal.
