They came with all the Panosh Palace action figures of the drivers, and one came with the castle. They weren't actually used for anything. The villain characters came with coffin nametags instead of the keys.
Dang you beat me to it TT. Yeah, the castle key is cool. I didnt know it was there for about 3 weeks after i got mine. "Hey whats this?" Imagine my surprise. Just like finding modes for transformers I never knew existed.