Astrid, I can assure you that I don't associate castles with Disney.
I live in Scandinavia, where the countries are kingdoms, so to me, castles are places where real kings, queens, princes and princesses live. And some old castles which for practical reasons are no longer inhabited by royals are elegant museums. If you are invited to a party in/at a castle here, this is usually a rather formal event, unless you are told otherwise. In YOUR part of Europe it may be different, but generally speaking, people seem to be more formally dressed in Central Europe than in Scandinavia. That's why I was surprised you would wear a denim jacket to a wedding in a castle.
When I think about it, I actually remember to have seen some German castles that looked like rather rustic places. And I'll admit that "your" dress code sounds informal, so you'll possibly be ok in your denim jacket though it isn't festive. However, you asked what we thought ...