Treasure Room
It didn't actually seem to take all that long in order to find the right place, and while the castle was surprisingly large, it wasn't like it was a labyrinth. The layout was quite straightforward, so Eisen managed to deduce the place where the crystal would be relatively easily. It would most likely be in the safest place that it could be, which was in the catacombs underneath the castle.
These, however, were in turn a bit more confusing, as they were supposed to be. It seemed like they were supposed to be an emergency escape route designed to confuse intruders. But luckily, with the help of some of Xenia's magic, they managed to find a quite large room pretty quickly, and it was directly in the center of the Labyrinth-like catacombs.