Not to miss. You have two possibilities. Free lunch concerts (you need to check the schedule on line beforehand as they're not on every day): don't let the long lines discourage you. There was one inside and another very long one outside but the concert is in the main concert hall that sits about 2,000 people. Still make sure you have your priorities right because it's a 30 minutes concert. The staff was very patient and accommodating after the concert and left plenty of time for photographs and never lost their cool even when a very enterprising tourist climbed on stage for a better view!
Regular concerts are great. (Bach's St Matthew and St John Passions when I was there). If there are no more seats in the stalls, the balcony is a good option. The second row is raked allowing for unobstructed views. Great acoustics. Great atmosphere. No particular dress code (from casual to soberly elegant).
The café on the premises is a good option for a pre concert meal. Not a big menu but something for everyone's tastes. Service is quick enough. It does get crowded and you may be offered to share a table.
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