Alright, finished on Hard Mode! Managed to beat Khattam-Shud as well (I haven't actually got to him yet because I need to go, but I have well over 25K health, 1100 ATK and an Invisibility Potion. Should do the trick).
So this means the hold is definitely conquerable on Hard! If you really need me to mention post-Khattam stats, I'll do so tomorrow or the day after, no time today.
Mind you this is a pretty optimized run (except for the last Level, I forgot to buy the VIP card and I plunged into the aumtlich trap with about 8K HP, so in total I could have had about 10K more HP in the end), which means that a LOT of effort was put into completing the first four Levels, which were very hard (after that not so much as it was not too hard to always keep my health at an acceptable level using the various keydrain rooms).
Later Levels are not as hard, except for the final one if you wish to make optimal use of your accessories and Lucky Blade. This means the difficulty curve is a bit weirdly shaped. In percentages of maximum difficulty:
SS: 25%
FT: 20%
DS 1: 100%
CC: 90%
DS 2: 50% (go for Long Sword and then 330 ATK or get to keydrain rooms?)
BS: 30% (only because of the Aumtliches. Otherwise it would have been 15%.)
RG: 55%
NF (including LTV): 90%
I don't know if you were still going to make any changes based on this report or not, but if so, I'd suggest leaving the first two Levels as they are. They're relatively easy yet good for warming up your perhaps still rusty mind. The Goblin might be made a tiny bit weaker though, if you want players who do not possess above average RPG skills to be able to pass DS part 1.
For the rest, I'd say it's all fine. I've said the later Levels are easy, but that's only if you optimized very well up to that point. Therefore I'd say it's a good reward for the player who puts lots of effort into every decision.
If you were not planning any changes, IT'S TIME TO PROMOTE THE HOLD!
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Don't worry, be happy, and slice monsters. You'll probably do better than me at all three.