Bielefeld is an east Westphalian city at the edge of the Teutoburg Forest. It is famous across Germany for a joke conspiracy theory claiming that it does not exist, which is unfair but useful: it tells you the city is not trying too hard to impress.
The real Bielefeld is a practical university and business city with one strong landmark. Sparrenberg Castle rises above the centre and gives the best reason to stop, with views across the roofs to the wooded ridge. Below it, the rebuilt old town has a few handsome squares and churches, though much was lost in the Second World War.
Bielefeld is not Cologne or Aachen, and it should not be sold as one. But if you are crossing eastern North Rhine-Westphalia, it has enough for a good spare day: the castle, Kunsthalle Bielefeld, the Dr. Oetker visitor centre, and quick access to forest walks above the city.