Baptistrery of San Giovanni in Fonte (Naples) (church building) built
400 CE.
Shown on the map at 40.853°, 14.259°.
The Baptistery of San Giovanni is a Paleochristian baptistery attached to the Santa Restituta church in Naples, Italy. It is the oldest surviving baptistery from medieval Europe.
Text from the Wikipedia article Baptistery of San Giovanni in Fonte, Naples, licensed CC BY-SA 4.0.