Perhaps the most interesting thing about the church is that it incorporates the remains of three temples of the Republican era (2nd century BC) which used to stand in a row, side by side in the ancient Forum Holitorium with their entrances facing east. [...]
The nave has seven ancient columns in the arcades on either side, and as mentioned they are not a matching set. Most are Doric, but the four nearest the presbyterium are Ionic. The flat 19th century nave ceiling is coffered in large panels, and is richly decorated in blue and gold with rosettes and tendrils. The coat of arms of Pope Pius IX is displayed. The ceiling of the transverse presbyterium is higher.
The high altar has a baldacchino, and beneath it is an antique green serpentine bath containing the relics of martyrs. [...] There is a U-shaped sotteraneo in front of the altar, with a marble balustrade. The apse behind the altar has frescoes from the 19th century restoration, and are of 1865. On a fluted column near the door, you can see the 11th century dedicatory inscription." (Text from the Churches of Rome Wiki).