The massive Romanesque bell-tower (1124) is crowned by a cusp, over which one may see the side of the church and the apse dating back to the 12th century. The interior of the basilica is divided by three naves with twentytwo antique granite columns with stucco capitals, added up during the restoration occurred in 1600, and was decorated by Pietro Cavallini, whose school also carried out the absidal mosaic. Two porphyry columns, the biggest ones in Rome, support the triumphal arch. The nice cosmatesque floor covers the entire surface of the church. In the seventeen-century ceiling with carved and golden coffers with a light blue background is a copy of the Thriumph of St Crisogono, a marvellous work by Guercino.
Remains from the first church, possibly from the reign of Constantine I, and earlier Roman houses can be seen in the lower parts, reached by a staircase in the sacristy. The ruins are confusing, but you can easily find the apse of the old church and you can see the remains of the martyr’s shrine in middle of the apse wall. The church had an uncommon form; rather than the normal basilical plan with a central nave and two aisles on the sides, it had a single nave.
On either side of the apse are rooms known as pastophoria, service rooms of a type uncommon in the West but normal in Eastern churches. The one on the right-hand side is thought to have been used as a diaconium, with functions resembling those of the sacristy in later churches. The other would then probably have been a protesis, where holy relics were kept.
A number of basins were found here during the excavations, including one cut into the south wall. As the plan is so atypical of early Roman churches, some believe that the structure originally had a different function, and the presence of the basins could mean that it was a fullonica, a laundry and dye-house. The area was a commercial district at the time, so this is quite likely. Others think that the basin in the south wall was made for baptism by immersion. As there were other basins too, it seems more likely that it was originally intended for a different use, but it may very well have been used as a baptismal font after the building had been consecrated as a church.