Early Christian baptisteries

In Christian architecture the baptistery is the separate, centrally planned building around the baptismal font. In the early church, catechumens were instructed there and the sacrament of baptism was given there (Wikipedia). From the 4th century, baptisteries in Italy were often built on an octagonal plan, a model that came from the Lateran Baptistery, the first structure built expressly as a baptistery; others were twelve-sided or round.

Baptisteries belong to a period when great numbers of adults were baptised by immersion. They were attached to the bishop’s cathedral, not to parish churches, because a bishop customarily baptised all the catechumens of his diocese, and the rite was performed only three times a year, on certain holy days. They had to be large for that reason, and some were large enough for councils to meet in them. Walls and domes were commonly decorated with frescoes or mosaics, often scenes from the life of John the Baptist. After the 9th century, as infant baptism became the rule, few new baptisteries were built.

The Baptistry of Neon in Ravenna is the one in this atlas: a late Roman building, plain on the outside, whose reason to visit is the mosaic in its dome. Its page says how to get in, what it costs and what the ticket also covers.