I'm keeping this as spoiler free as possible in case someone completely new happens to read just this review, but after the following paragraph I will talk vaguely about the mood and structure.
I'm somehow very new into FMs, only having gotten into them this past week despite having been Thief fan for half my life. And what a mission for one of my first ones, and the first to complete. Superbly well written and crafted, intellectually challenging but above all *fair*. I admit I had to look to the forums for hints at few times, but in retrospect I had all the information I needed right there with me, I was just too impatient. There was only one bit where I had no idea I even could do something I needed to in order to find a vital clue, since the base game hadn't trained me for that approach, but it's possible I had also missed a hint somewhere in the mission.
This is the first game experience in a while to give me some actual horror feeling, too. I'm sick and tired of jumpscares, and while there was one part in the mission that might qualify as one, it wasn't very obnoxious and didn't rub my nerves the wrong way. The rest of the time was pleasantly unnerving, with some Thief horror staples used smartly and sparingly and most of the horror coming from writing and the mood.
So too was the experience of puzzle solving very satisfying. Like I said, it feels very fair, expecting you to (mostly) use skills you learned in the main Thief game(s), only much more intensely, and to consider carefully all the clues you find. The map isn't super huge, which feels strange to realize now right after completing it, since it feels so intricate and full of stuff, an onion of mystery.
I think the size is rather perfect given the amount of going back and forth there is, *especially* if you are going to ghost it. I myself took the easier approach of knocking people out whenever possible to make exploration easier, and so the whole mission took me 3 hours 22 minutes, a lot of which was spend carefully looking at places I had already been to, in case I missed something (and often I had). Ghosting it should be very much possible and very fun for those so inclined.
Only bug I can remember is having trouble entering a shallow crawlspace in one point, but it's not super vital spot and it always worked after few tries. Otherwise the mission appeared bug free, which is remarkable for something this intricate.