Adrian Su
Google
I recommend purchasing your ticket online as it is impossible to buy it on the spot. Although there is a qr code that you can scan at the meeting point (on a small plaque on the wall of the old city hall, very inconspicuous), each tour is limited to 20 people so advance booking is still the best option. There is only one English guided tour at 18.00. If you end up in a German guided tour and don't understand much German, tell your guide at the beginning - - they will give you an audio guide. It doesn't give as much as information as the guide themselves, but at least you won't be completely lost.
The tour takes about 50 minutes during which one goes through a series of dungeon chambers with the guide. It is a very cool place; one can learn quite some history down there.