The search doesn't take that long, and you're not disturbed while conducting it. Once Rah points out the first mark on a flagstone - a red hand pointing downwards - finding the rest are quite easy. They lead around the foundation of this ruined structure, to a small building about twenty feet from the actually castle. This might be a good thing, considering the state of the castle proper. It looks like it's been heavily damaged in some sort of battle, and trying to search within it looks like it would be a deadly affair.
Finding the structure takes roughly twenty minutes of your time. The door is wooden and perhaps magically enchanted to resist rotting, as it stands as solidly now as the day it was constructed. An inscription is written above the door: "Within lie the nobles of Rivenroar, revered in life, revered in death." The latch seems quite simple to operate - a simple sliding mechanism accessible from either side. Probably simply for privacy, as it wouldn't keep graverobbers out.