Shuttle bus from Ravenna to Ravenna (city)
Most cruise lines provide a shuttle bus (whether or not complimentary) from the cruise port to the center of Ravenna. They take you to the Viale Farini, a boulevard that leads to the train station. This is a central location within walking distance of the highlights.
TIP: Check the price (if any) in advance, because there's a very good alternative by public bus.