Man, that was WAY bigger than I expected it to be. Huge, even.
Overall I liked it and it had lots of character, which I appreciate more than perfect polish. It's great when authors try something new.
I liked the pagans. The pagans are great. I would like to see more pagans. The "parallel worlds" created by pagan shortcuts vs the rest of the city were great as well.
Story was really nice.
The keys were a solid idea. I wouldn't want all (or most) missions to be like that, but once in a while, why not, it provides a different experience, which is good. The only issue I have with it is that I think you may have underestimated how trivial it is to overlook a key even if it's not hidden in any way, and I think people have biases about where and when to seek keys from other missions. I think that from past experience (especially in various city missions) I'm not used to looking for relatively crucial items in areas that are not necessary for finishing a quest objective. Don't know if this is really your fault, but I'd say I'm not the only one this happened to.
For example I missed the gate key completely because I simply forgot to look around in that particular small piece of the mission, I just discovered a different crucial key (that let me leave the area) pretty much right next to it and I didn't see any mission context that would suggest "this is the area where the map finally opens up and you should find the gate key". The key is not necessary to finish, but not having it makes the final part a lot less fun.
I liked that some things that happened were pretty unique. Raiding the chief builder's quarters by just running through everything was an absolute carnage and funny as hell:
Now some criticism:
First one is really superficial, but I didn't look at your mission for a while because you chose one of the most generic names possible and I think I subconsciously assumed the mission itself is not going to be a great effort either (in hindsight: LOL!). If I had to say one single thing I actually dislike about the mission, it's the name.
Some details here and there, like a door in the Ox 2nd floor (I think? across the street from the hypos) completely missing and only showing a wall and a levitating doorknob in the doorframe, some levitating vegetable garden on the farm or misaligned branches here and there, nothing important.
It was also possible to look out of bounds in several places, but the only one where I think it was an issue was in the square where the pub is, next to the Builder's building - there's a scaffolding that you climb on, you can look through a semi-open window and there's a ledge nearby that you can climb on. For me the place screamed "climb onto this and see where it goes", only to show me an out of bounds view and a z-fighting texture.
It was a tad too long and the pacing around the end was not great.
Length itself would not be an issue I think, but after finishing the main mission it took me another hour to complete the medium loot objective. I approached the grand finale, fulfilled the main objective, and instead of victoriously arriving with the cure I had to roam around for an hour looking for places that I missed. And I did miss some, obviously, but that's simply often going to happen with a mission like this. That was more tiring than pleasant.
The biggest problem for me: Lighting.
Seems like there was light shining through walls everywhere. Light gem was shining in a few places that seemed dark. Places that should have been dark (no windows or other light sources) weren't and instead seemingly contained invisible light sources that didn't make sense (the room on the ground floor of the bleak house was like this I believe). I noticed that some light fixtures continued to emit a little bit of light even after being extinguished, which is not an issue, I'm talking about places where I could not find such rationale for the light.
I'm surprised that nobody complained about this yet, almost makes me wonder if there's something wrong with my installation, so I took a few screenshots to show what I mean: https://imgur.com/a/JuO9sF5
If this is really how it's supposed to be, then this is the one thing I'd personally focus on improving in your next mission, which I'm sure is going to be great.