The Arian Baptistry is small, plain and easy to miss, tucked on a side street east of the old centre, and that is half of its appeal. It is an octagon of brick, built by Theodoric in the early sixth century for the Arian Christians, the faith the Goths followed. Arians believed that Christ was subordinate to God the Father, a view the orthodox church condemned as heresy, and so Theodoric's Ravenna had two communities, two cathedrals and two baptisteries, living side by side with surprising tolerance for an age of war.
Inside the dome, the mosaic shows Christ's baptism in the river Jordan. Christ stands naked in the water while John the Baptist pours water on his head, and a river god sits beside them holding a reed, a pagan figure borrowed from Roman art that lingers in a Christian scene. Around the rim the twelve apostles walk in a circle, in procession towards an empty throne. It is compact and strangely moving, and you can take it all in within ten minutes.
When the Byzantine army took Ravenna, the Arian buildings were seized and rededicated, and the baptistry became a Catholic chapel. The mosaics survived, and parts of the original decoration were restored. The ground in Ravenna has risen with centuries of silt while the buildings stayed put, so you often step down into these old rooms rather than up.
For riding, the nearest good reward is Baraccano, a Bologna landmark on the route list, reached across the Via Emilia, an hour away. Between the two, the Po plain is as flat as a table, and the roads, canals and drainage ditches run in straight lines. Fog in November can hide a cyclist at ten metres, so slow down.
Fuel is available all around the city, and the old town is a short walk. Stop for a piadina or a coffee on the Via di Roma and give the baptistry a quiet moment before the next building.