We visited this lovely castle when we were in Thun anyway and realized that whatever we had wanted to do wasn't on the day we were there. A lovely coicidence, because visiting the castle was a very welcome replacement. I think I enjoyed this trip more than I would have with the original program.
This is what the website of the Schloss Thun has to say about it:
The mediaeval castle above the town of Thun is a prominent eye catcher at the entrance to the Bernese Oberland Region. The complex was built by the Dukes of Zähringen at the beginning of the 12th century. Located in the almost unaltered preserved dungeon, the castle tower, is the so called Knight’s Hall, one of the best conserved representation halls of the Middle Ages in Europe. The corner towers allow for an unequalled view over the Alps, lake and town. The castle’s five storeys house the historic museum, which features the regional history as well as special exhibits on different cultural-historical and current topics.