First time making a "serious" dungeon! How do you do it?

[Quickleaf]

This is fantastic stuff. What I like, is that you not only detail what the area looks like, but also the behavior of its denizens, which informs their patrol routes.

Thanks. Yeah, having some grasp of how monsters dynamically inhabit a space is really important to support player creativity.

If the players decide to infiltrate the slaves in the sulfur mines and sneak through the main gate when the slaves are recalled for meals, then it helps to know that slaves bearing sulfur are taken by guards to the alchemy lab where that sulfur will be used.

One of the things that I often do when designing a dungeon, is come up with minor 'discoveries' that the players can make, that are not bound to any specific room in the dungeon. For example, they could discover an ancient stone mural that reveals some important bit of lore. I then keep these blurps of text in reserve, for when I feel they are appropriate. I can just throw them into any location whenever I feel like it. That way I can keep every moment in the dungeon exciting. Even if they just stop to rest somewhere, and take in their surroundings, they may spot something that they didn't notice before.

For example, for the sunken coastal city I had these bits of lore ready, that I could drop in at any time:

I love those evocative examples! They go a long way towards evoking the feel of the sunken dungeon you created.

Having little random descriptions like that is a great idea. Usually I end up imagining/writing specific areas, but the truth is that my players enjoy exploring on their own terms. I might have a great description of a room prepared and they might not care at all because they're more interested in teasing the halfling PC for some cursed object he has. And I never know when some small detail is going to inspire them, immersing them into a scene where they'll want more details. I think your approach here really embraces that sort of organic waning/waxing of focus and facilitates players going off on exploration tangents.

I also appreciate how you include little traps/tricks/treasures into the mix. Consider the approach borrowed