I want to start off by saying that this is an amazing fan mission. The atmosphere and level design are great, and the new enemies, while basically being reskins, are still incredibly well done. The first time you see one of those giant, pale-white, red-eyed, sharp-clawed yeti things, you will need a change of pants. And that's just getting things started.

Atmosphere is one of the most important things to get right for a good thief mission, and this one nails it with particular aplomb. The environment, the architecture, the creative use of game assets and the dramatic overhaul of enemies is all, 100% phenomenal. Chef's kiss.

However... Navigating can get a little confusing. You get a map, but it's about as helpful as most maps in thief, and since a large part of this mission is key-hunting, you'll be very hard pressed to find your way around for a while. This might be intentional, and I'll explain why.

While this mission doesn't explicitly force you to ghost it, you're going to have to ghost a LOT of it just because of the sheer number, variety, and types of enemies. You might go into it thinking 'oh, it'll have lots of undead, better stock up on flash bombs.'
Do not do this. Do not. No. Because undead are only a fraction of your problem.

Some enemies can be sapped and some cannot. You will not know which is which ahead of time. And the enemies range from incredibly creepy to 'have to go to therapy now,' and you will not have the resources to deal with them. Even with the generous cash loadout to begin the mission, you'll need to think very carefully before even loosing a broadhead arrow.
You might think, oh, it's just a spider, let me take it out. But then the dead spider spawns 3 live spiders. So now, even if you hit with perfect shots, your one arrow shot has become four.

Some humanoid figures are undead, some are not. Some have a secondary poison gas detonation when you knock them out. Some are made of bloody rock, but thankfully your arrows do damage.
And the yeti things are too damned fast!
Luckily the monsters aren't necessarily all on the same team, and I solved a lot of my problems by luring them towards each other and letting them have it out. Even still, I got my butt kicked fairly hard.

So you'll likely be ghosting through most of this, like I did, because I made a few bad calls early on and spent most of the mission with 1 HP left and a massive fear of heights.
And in typical Thief fashion, the way you thought you were going to leave through is taken away at the last second and you have to ghost your way back through other stuff to hunt for a key at the spider shrine. No, not that one, or the other one, but the one up above that one with even more spiders.

I'm old hat at Thief, but this took me over 3 hours. The puzzles themselves aren't too difficult, and as difficult as this mission is, I don't feel like it was ever unfair or badly done. Just be aware when you go into this mission, it's not going to be easy even on Normal skill.

But if you love creepy settings and challenging missions, this is 100% worth it.