Lucky solver #12!
Just this morning, I finished the last few rooms of Level 14 of Bavato's Dungeon and escaped, making me what looks like the 12th person to actually conquer this hold.  
It was quite a wild ride.
I did leave a few loose ends behind, though:
2 non-required rooms on Level 10
The entire Force Arrow Maze (Level 12)
1 non-required room on Level 13
I'll probably try to polish those off soon.  It's either that or go back to that secret tar room in Beethro's Teacher (shudder).
Level-by-level impressions:
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Level 1 (The Security Gates):  There wasn't a lot to this level.  Most horde management and not-overly-complex orb puzzles.  I was worried that the entire hold would be like this...fortunately, I forged on.
Level 2 (The Storage Chambers): Some awesome (and difficult) puzzles on this level.  My favorites were 1S3E, 1S and 2S1E.  1S stumped me for quite a while...
Level 3 (The Prison Cells):  And here I came to a screeching halt.  Two rooms that require efficient tar-clearing (1N and 1N3E) bested me, even with hints, for months and months.  I hung it up and went on to other things.  Most of the rest of the rooms on this level were more horde management -- not much puzzling to do here.  I came back eventually and managed to clear them and move on to...
Level 4 (Above The Pits):  A lot of difficult puzzles involving precise manipulation of goblins or wraithwings.  I thoroughly enjoyed this level, though.  Favorite rooms:  1W, 1N.  Biggest stumper: 2E.
Level 5 (The Tar Flood): Most of this level was not too bad.  A good mix of tar cutting and tarbaby management.  Best rooms: 1E, 1S1E.  Hardest room: 1S4E.
Level 6 (The Rooms of Manipulation):  Some real stumpers here.  Two of the rooms on this level (1S1E, 1N2E) completely stumped me for the longest time.  Favorites: 1N, 1S2E
Level 7 (The Collapsing Floor):  Utter craziness.  Some of the hardest rooms in the hold (or maybe I just have a hard time with trapdoor puzzles).  No favorites on this level, but 1N2E and 1S3E were both very difficult.
Level 8 (The Ruins):  A more relaxing level after Level 7.  Lots of nasty hordes, but nothing too difficult.  I left the two-mimic puzzle for last, and it turned out to not be as bad as I feared.  No big standouts -- all rooms were pretty even for difficulty, all were pretty good.
Level 9 (The Floodgates): Here the difficulty cranks up a notch or two.  Most of the rooms on this level are very tough.  Efficiency and careful planning a must.  Favorites: 1N1W, 1N4E.  Hardest room: 2N (although 1N2E is a very close second).  All in all, a very tough level.
Level 10 (The Halls of Mimicry):  Some brutal mimic puzzles.  I didn't even complete the two non-required rooms on this level...I quite liked 1S2E and 1S4E.  Hardest room (that I completed): 1S4E.  
Level 11 (The Fissure):  A story level.  Some puzzles, but nothing too diffcult.
Level 12 (The Force Arrow Maze):  OK, I skipped this.  I did map it out and try to find a path, but ultimately I decided that I'd rather move on.  So, moved on to:
Level 13 (The Stronghold):  Certainly where all the hardest rooms come to train.  Apart from a couple of filler rooms, every room on this level is a real design masterpiece.  Difficult and elegant.  Hardest rooms: 3E, 1S5E.  I didn't beat the non-required version.  I'm glad agaricus went with the easier version, though.
Level 14 (The Escape): Somewhat anticlimactic after the high level of difficulty of Level 13, I was able to breeze through these rooms with both my kids sitting on my lap.  
Overall, an excellent experience.  I give this hold a 10, with 9 brains of difficulty.
I look forward to (and dread) the next hold agaricus produces...
Josh
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"
Rings and knots of joy and grief, all interlaced and locking."
  --William Buck