Roman ruins aside, this is Ravenna's oldest intact building, constructed over the site of a former Roman bathing complex in the late 4th century. Built in an octagonal shape, as was the custom with all Christian baptisteries of this period, it was originally attached to a church (since destroyed). The mosaics, which thematically depict Christ being baptised by St John the Baptist in the River Jordan, were added at the end of the 5th century.
Note that visits are sometimes not allowed due to religious services.