At the main entrance of the Duomo, 3,8m (12ft) below the parvis, you’ll see the Baptistery of San Giovanni alle Fonti and Santa Tecla’s Apse. These ruins have been discovered in 1889 and are essential for Milan’s history and identity. The Baptistery of San Giovanni was built between 378 A.D. and 397 A.D. amid the cathedrals of Santa Maria Maggiore and of Santa Tecla, on the area that today is occupied by the Duomo’s façade. Here, on Easter’s eve on 24th April 387, Saint Ambrose baptized Saint Augustine, who had converted himself to orthodox Christianity.
Its octagonal basis may have been influenced by the Roman mausoleum and reminds the eight characteristics needed in order to be blessed and the seven days during which the world has been created plus the eighth day of the eternity; the eight rectangular and semicircular niches, overlooking the sides of the large central font, are very interesting.