One of the oddest dungeons I've come across as far as balancing. Most of it's surely quite hard, but only a bit of it rewards XP and loot suitable to the level required (in my opinion is 12+ as a cleric). Mind you I'm only talking about the basement and 1st floor of the keep, and the towers on the walls. I also have only done this dungeon a few times at level 10, once at level 11, and once at level 13.
The first and most common enemy are the Ghostly Armors; they not only cast ghostly visage, but inherently have 50% concealment. While that isn't an issue alone (as most people will be buffed with a +2 enhancement at least when attempting this dungeon), they also have a ridiculously high AC; they become almost impossible to hit even at mid-high levels without cleric-like AB spikes and/or haste. Eventually, you become subject to their nasty crits as they outlast you. The XP they give seems absolutely absent given how high of a level (because of the AB needed) you need to be.
The second and most common "enemy" are traps; some are quite easy to ignore as they barely inflict much damage, but the actual good loot requires someone capable of disabling the trap or you risk instantly dying to an electric trap. While not an issue in itself, this means that this dungeon now requires a significant party to get through; a dungeon that offers little significant loot outside of two chests, and no XP unless you're around level 10 (from my experience).
Otherwise, this is a great dungeon; the atmosphere, the white texts, the history, and the bit of story strung about in the letters. It feels like a flavor dungeon, like the small temple/keep with the dark orb just nearby this keep. If it was intended to be just a dungeon for flavor, then it should be dialed back a good amount. If it's both one for flavor and an actually repeatable dungeon, then up the rewards.
Now, if my past parties and me are doing something wrong (we took a very linear approach), then please do just mention that though don't spoil the details. Otherwise, I feel that barely anyone cares to run this dungeon given the risk vs. reward. I pray I'm proven wrong, because this is such a good dungeon visually.