Stock im Eisen is a small but memorable medieval curiosity beside Stephansplatz: the preserved middle section of a tree trunk hammered full of nails. Travellers and apprentices once drove nails into it for luck, thanks, or ritual obligation, leaving behind a dark, spiked relic in the middle of polished central Vienna.
It takes only a minute to see, but it is worth noticing if you are already at Stephansdom. The legends around it give the old city a stranger texture than its grand churches and imperial facades usually suggest.