There has been some kind of church since she was martyred in the middle of the 3rd century CE, and the church today dates back to the 7th century. But the remarkable thing is how the church is built below the road (or the road is built above the church). From the road you see the top of the apse. To get to the entrance you take a long staircase down into the earth, just above her grave and the a network of catacombs (which we also visited -- stone bunk beds for the dead, five high on either side of the tunnels).
Nearby is the circular church of Santa Constanza, the daughter (or granddaughter -- some dispute) of Constantine who took on the protection of the tomb of Saint Agnes. It is a remarkable structure from the 4th century, its dome undisturbed -- a Pantheon in miniature, but far less visited. Perhaps the most stunning element, however, is not the dome but the complete sing of mosaics, which circle the inner sanctuary. These mosaics have no overtly Christian content, as the church was built so early that pagan elements were the norm. Winemaking seems to be the dominant theme.
To leave this moving early church and walk ten minutes to Mussolini's home during much of the 1920s and 1930s was quite a shock. It was made both easier and more disturbing by the fact that the information panels of this 18th century mansion (now a public park and soon, I am told, a site for a Holocaust memorial) say virtually nothing about Mussolini. We get to his private rooms, and the sign says: "This was Mussolini's bedroom." They are much more interested in showing off the home as it was built by Prince Torlonia.
I later found out, at a party at Eric Salerno's house, that Mussolini's grandson (son of his daughter, a right-wing elected representative) goes to school at St. George's English school, which sits immediately next to the Villa Torlonia.