Santa Costanza is a 4th-century church on the Via Nomentana, which runs north-east out of the city. It is a round building with well preserved original layout and mosaics. It has been built adjacent to a horseshoe-shaped church, now in ruins, which has been identified as the initial 4th-century cemeterial basilica of Saint Agnes. Santa Costanza and the old Saint Agnes were both constructed over the earlier catacombs in which Saint Agnes is believed to be buried. According to the traditional view, Santa Costanza was built under Constantine I as a mausoleum for his daughter Constantina, later also known as Constantia or Costanza, who died in AD 354.