I dropped this name only to make sure that, free doesn't mean it is low quality. (As I was, perhaps, expecting myself). My biggest regret is that I do not have enough time for attending as much as I would like to this wonderful musical gathering. There are lots of tourists, but also people working in the area - white collars - students, children, retired persons - from 1 month - music lovers coming regularly. If you arrive a couple of minutes before it starts (as I did this week), you risk to have the worse possible view, but still the sound is good and you can fully enjoy the musical performances. The musicians are young or vetarans, all of them professionals and sharing wonderful moments with a very enthusiastic and polite audience.
Here are my visual snapshots from the last event, with a repertoire from Claude Debussy, Bach, Per Nørgård, Jacob Druckman, Max Leth and Iannis Xenakis, in the interpretation of Simon Roessler and Wieland Welzel.