The Basilica di Santa Maria in Trastevere is one of Rome’s oldest churches, originally built in the 4th century. While the structure has been renovated and expanded upon since then - most notably in the 12th century, when it was essentially torn to the foundation and rebuilt - the floor plan still reflects its 4th century roots.
Although there is some dispute as to which was the first church in Rome dedicated to Mary, there is an inscription in the Basilica di Santa Maria in Trastevere that indicates this is the first such church. The original church on this spot was built in 340 under Pope Julius I, and in the 1140s Pope Innocent II tore it down in order to rebuild it completely.
The Basilica di Santa Maria in Trastevere is noted for the shimmering 13th century mosaics in its interior, and its Basilica designation comes from the fact that it is home to a relic of a saint. In this case, it’s the head of Saint Apollonia. There are also tombs of two popes in the church - including Pope Innocent II. The mosaics on the exterior are from the 12th century, when the church was rebuilt.