Nonnebakken is the site of a Viking Age ring fortress in central Odense, one of the Trelleborg-type fortifications linked to Harald Bluetooth's kingdom. The surviving earthworks are modest, but the site matters because it belongs to the group of Danish Viking ring forts listed by UNESCO.
Come for context rather than spectacle. It works best as a short stop while walking through central Odense, especially if you are interested in the city's much older history beneath the Hans Christian Andersen layer.