Gråbrødrekloster Museum is an underground museum beneath Algade, built around the remains of Aalborg's medieval Franciscan monastery. The exhibits place the monastery beside the old market place, lost churches, and town houses that once shaped this part of the city.
It is a compact stop, but a good one if you want to understand the layers under Aalborg's shopping streets. The entrance is easy to miss, so look for the museum access in the centre of town rather than expecting a large building above ground.