As the other replies have pointed out, there aren't really undiscovered castles.
But I do recommend visiting Germany, where there are way, way more castles than you might expect. In particular I can recommend the "Castle Way" along the Neckar River. Every bend in the river has its own castle, just dozens and dozens of them.
Most are not especially dramatic. Many are just ruins; others are literally just people's homes now. Still, it's a little bit like a D&D fantasy world where there really are just castles everywhere. (And yeah, some of them have genuine dungeons.)
But I do recommend visiting Germany, where there are way, way more castles than you might expect. In particular I can recommend the "Castle Way" along the Neckar River. Every bend in the river has its own castle, just dozens and dozens of them.
Most are not especially dramatic. Many are just ruins; others are literally just people's homes now. Still, it's a little bit like a D&D fantasy world where there really are just castles everywhere. (And yeah, some of them have genuine dungeons.)