When the walled town was first built it was quite small and no one was allowed to build outside the walls; the enemy might use the houses for hiding places. There was a small river meandering through the city and it only seemed logical to build over it. The underground river was originally just sewers but, after the advent of sewage treatment, the passages were improved and the result was kind of a Disney'esque under-the-city boat ride. Unfortunately the narration was all in Dutch so we could only get small bits of what was talked about but it was an hour well spent.
This is the guest dock in town. When we arrived on Thursday it was full and we had to raft up alongside another boat. By Tuesday it was pretty quiet.