Santa Maria in Trastevere.
Founded in the 3rd century, this church was one of the very first official places of Christian worship in Rome. The present building dates mostly from the 12th century. As often happened because of the Romans' innate efficiency, the granite pillars in the nave were taken from the ruins of various ancient temples and are indeed all different. The superb 12th century apse mosaic depicts the Coronation of the Virgin. The pavement is a 1870s reconstruction of a 13th century Cosmatesque mosaic floor.