An opening leads off from the gallery into a foundation area formed by large retaining walls supporting the upper floor of the Tabularium.
At ground floor level we can see the remains of a building which predates the Tabularium.
One of the three rooms which have been preserved, contains one of the oldest mosaic floors ound in Rome, a white background with irregular chips of coloured marble.
The Tabularium was preceded by older buildings, constructed on multiple levels along the slope of the hill and probably destroyed by fire in 83 B.C., the same that destroyed the temple of Jupiter and much of the surrounding area.
The Basement room preserves the remains of mosaic and cocciopesto floors of the late second century B.C.
Overlooking the modern wooden staircase one can also see a central room, enriched by a fine white mosaic with black borders, which opened to another room, or terrace, with a floor composed of fragments of coloured stones on a white background ( lithostroton).
The remains of the floors of the Republican age hang on the wall opposite the entrance come, instead, from the immediate vicinity of the temple Vejovis.
A metallic net indicates the presence of a cistern covered with cocciopesto, as shown by recent excavations and attributed to an earlier period.