The Tabularium In antiquity, the Capitoline Hill had two high points, the Capitolium to the south and the Arx to the north, separated by a saddle (known as inter duos lucos or Asylum). The building known as the Tabularium was constructed in this area, among the remains of more ancient buildings, overlooking the level part of the Forum. Its trapezoid plan occupies the space between the Capitoline rise (Clivus Capitolinus)  which is the ancient road to the Temple of Jupiter on the Capitolium and an extension of the Sacra Via and the steps (Scalae Gemoniae and Gradus Monetae) leading to the Arx (Aracoeli) and the Temple of Juno Moneta.

The site intermediate between the two summits and land conformation determined the configuration of this imposing building and the complexity of its functions in the course of time. Much of the building remains today under the Palazzo Senatorio. The Tabularium, identified as such by two inscriptions and a transcription that identify the consul Quintus Lutatius Catulus as the person who ordered its construction, Lucius Cornelius as the architect, and 78 BC as the probable date of construction, has the recognized function of substructure forming a terrace on sloping terrain.

The Tabularium therefore consolidates and contains the area of the Asylum, as well as defining an edge of the Forum. The basement measures 73 meters and is designed to support various upper floors, which it has continued to do well beyond the life of the Tabularium itself. The function of the Tabularium is less clear: the idea that it housed the state archives is controversial. Indeed, centralization of the functions of conservation and consultation, as well as record-keeping in the current sense, were unknown in late republican Rome. For example, documents were kept in the tablinum, the private archives of patrician families; they were entrusted to the protection of a god and deposited in temples, as occurred in the Forum; public acts were conserved and displayed on bronze tabulae on the walls of public buildings.

So although the histories of the Temple of Saturn and the Tabularium indicate a relationship with regard to the conservation and administration of documents, it is difficult to think of the Tabularium solely as a centralized archive. Rather, it is a complex building, both in its meanings and its uses: it is the architectural solution to an engineering problem and the monumental backdrop of a particular area of the Forum, of which it marks an edge. It was the focal point of perspectives from the Sacra Via towards the Capitoline Hill, at least until the Temple of Vespasian and Titus was erected, and a sophisticated multilevel device linking different spaces in the area of the Asylum through a sequence of covered public spaces (steps, passages, loggias). The basement, as it appears today, is made of regular blocks of tuff and peperino.

The wall has a series of small windows into the rooms on the first floor, reached by a narrow staircase, still present. The staircase in the basement had a door into the Forum which was walled up towards the end of the first century AD, on construction of the Temple of Vespasian and Titus. Three of the eleven arches are still visible on the facade of the next floor. The arches are framed by Doric half columns in peperino with capitals and architraves in travertine; there are also fragments of friezes with triglyphs and metopes. The arches flanked a long internal gallery, divided into pavilion-vaulted sectors to form a covered passage for the rooms that opened onto it, probably linking the two parts of the hill (the branch of the Clivus Capitolinus corresponding to the present via del Campidoglio, and the Gradus Monetae, now via di San Pietro in Carcere).

Scholars have different hypotheses regarding the next level, which no longer exists. It may have been a platform for one or more temples or a sequence of spaces organized from a portico, possibly Corinthian, superimposed on the underlying one that opens onto the Forum. Reconstructions consider findings from the small pre-existing Temple of Veiovis, that would explain the recess in the SW side of the building, and the remains of Corinthian capitals recovered at the base of the Tabularium, now displayed in front of the Portico of the Harmonious Gods. The forms, spaces and meanings remain a puzzle: the building was modified for various uses that ensured its conservation (prison, salt cellar, medieval fort, senate house), emphasizing the role of the substructure in the course of time.