Okay, so first thing I got in my overview pass was just how much this felt like a big wall of text. 3 columns, some bolding, and the exterior line wrap do a bit to help mitigate this. That said, some call-out boxes, uses of color, or other methods would have gone a long way to create some visual interest through the piece without costing any of the content. Incorporating the credits and link into the border were a smart use of an otherwise decorative element.
Page one is our setup and while some of the wordings could be tightened up a bit for better flow, clarity, or simplicity, it does it's job well. Several times I found myself quickly jumping down to skim what was being referenced to be sure what I wanted to know was going to be there. That's not an issue (especially with only 4 pages) though it could be if you decide to expand this. Something to be aware of.
At the mentioning of masks, it points out premade vs create your own. Jumping down, the area is pretty strait forward about what is there and obviously no mechanics since it is system agnostic. This said, I would have liked to jump down and find a few suggestions on existing masks (I can make up more details myself) to pull from in a pinch as a DM if I wanted a few there for players who aren't crafty (system dependent) enough to make their own and I just pulled this out of a folder for a one-off on a given night. System agnostic means no way to suggest a benefit to making their own, but I can figure that out on the fly from what is here.
Map threw me for a few minutes. My brain kept wanting to overlay it on top of the castle due to the similar shape and those guard posts (assuming they flared at the top) even though I know the page one and key said courtyard. This wasn't helped by the widest point of the map being the narrow part of the of the isometric visual and the actual squares being roughly a square when I counted them out! This might be something that could be addressed to improve the playability. I could also see putting a faint grey castle shape up against the proper wall so it's very clear how it is laid out in relation to the picture in the corner. If I am going in cold from my emergency game folder, this removes any pause in the flow for me.
I'm not a huge fan of flavor text paragraphs since I basically never read them and just roleplay out whatever's needed. In this case, I don't mind it a lot. It is a few sentences that inform me what is even going on here right at the start and not used again. Good. Easy to work with even if I don't use it as empty narration.
Breaking out of the cell being automatic and the roll being about not drawing attention is a good choice. Still might end in TPK given their lack of armor and weapons, but still solid choice. No risk of the dreaded 'everyone fails and nothing happens' some modules suffer from. Torture room is just a visual in theory, but players will almost certainly gain improvised weapons before moving on. Good choice for progression. The corridor is a bit strange. I'd have rather seen a set encounter here like a lazy guard rather than some of the strangely random options instead. The idea of giant rats (without any obvious source in such an actively used castle) or festival litter down below feels a little too 'this is a dungeon lol' to me. Save the table for later on and just put a lazy guard here not expecting to encounter anyone. Maybe separating the corridor below from the ones above in terms of your table and then replace rats with some castle dogs or something?
Nice choice again for a decision for the obvious exit or risking a threat to gain better armor and weapons. Masks I already covered, but good that there's no way to miss this location. Store room feels a bit dull, but depending on the roll could create some interesting events (system used making a lot of difference here). Courtyard's fine, lots of potential story elements or encounters. 'I' and 'J' look nearly identical on the map! I had to stare a few minutes when I had it at printer scale to decide which was which. Not helped by the fact that they are in similar placement. My original thought was it was 2 'I' locations serving to guard both ways to the gate, but apparently not. This also makes me question why there's only a gatehouse on one side and not the other.
The clockwork guard is interesting. It's not letting /any/ guest leave. This means players can try going through the castle, climbing walls, finding a way to get it distracted on another guest, etc. Player choice for the win. It's not clear if the players will be aware of its nature or if that's DM only info. And of course, this is meant to be able to plug into a larger setup. That's good too. Instead of a one-off location they might manage to finish in an hour, it can lead to several nights of play depending on the player choices and all I need to do is grab a castle map. If you decide to expand this, I would love to see the greater castle and maybe include some court intrigues in it. That falling body could even become a major story plot in some games.
I think the weakest element here is theme. It mentions a book (that never existed), which is kind of a fun meta, but in two years when I pull this out I am going to have no idea what that's about and since the book doesn't really exist, I am going to be left blinking and just ignoring that part. There's nothing here that really feels like it isn't just as easily an entry in every other dungeon map jam outside of that small note. I don't see any recurring characters who feel like I read them in a pulp fantasy novel. I don't have a sense of the greater world beyond 'everyone gets locked in and has to wear masks because reasons'. It's a solid dungeon. Is it telling me Appendix N if I didn't read it on the border of every page though?