While the volume of old documents is great, the manual research is a very slow, daunting process. However, one thing our resident researcher may have stumbled across, that could have some bearing on recent events, is the documentation of a fairly common practice, with a slight twist:
It seems that some 400 years ago the inhabitants of this castle anticipated an overwhelming attack, by a neighboring clan. Along with all the other preparations for battle, it was ordered that all the valuables in the castle be hidden where no one would think to look; In the crypts, beneath the castle.
the people who were tasked with this chore had actually removed the bodies from the crypt, to make room for the valuables to be stored there. It is said that the bodies were unceremoniously strewn about, outside the castle gates in a meager attempt to frighten off the impending invaders.
These are the facts, so far, as uncovered by your research. The types of documents this info was pieced together from is up to the researcher who compiled the information. No mention was made as to where the entrance to the crypts is/was located, but the phrase "under the castle" was used.