Currently getting back on track, making a lot of nice progress, especially with regards to level design.
I finally found a proper workflow, that allows fast design iterations, which was really important to me. Level design in basic Unreal is a bit of a slog, since the engine’s core tools are a bit limited.
Obviously Monomyth is a game focused on interior levels, so it needs a different bunch of tools - something closer to classic level design as seen in games like Quake or Thief.
As mentioned before I’m using Level Buddy (a blender plugin for Doom/Quake styled level design), but the real key to classic level design is the combination of level buddy with an Unreal Plugin called Mesh Tools.

On their own these tools are neat, but each one of them comes with a couple of drawbacks. Level Buddy crushes your texturing every time you rebuild your map and Mesh Tools - at least to my knowledge - doesn’t have those comfortable boolean unification options (i.e. you create, for example, two cubes and unify them into one mesh, leaving you with a new shape - this is bacially how level buddy works). Together however they are perfect for what I’m doing. Level Buddy allows fast blockouts/geometry creation and Mesh tools allows fast (in-engine) UV projection (that also includes texture scaling - which is bothersome if you have to switch between Blender and Unreal all the time). So my level design workflow currently looks like this:

1) Gathering reference material
2) A really quick doodle of the area’s outline.
3) Blockout in Level Buddy
4) Import into Unreal
5) Testing, cutting stuff, changing it again, bascially going back to 3) a couple of times - in this phase the design from 2) changes a lot. I tend to combine this with basic Unreal block brushes to roughly measure the scale of an area.
6) Once I’m confident about a part of the map, I snip that part off in level buddy and separate it into its own mesh.
7) Then I work over that isolated part again. I also do a quick UV projection (texturing) with Mesh Tools - this is like two or three clicks, so I can still change the level in Level Buddy afterwards
8) Then I continue with various detail passes - especially placing modular assets (like wall decorations in a castle or pillars in a cave)
9) I do this for every part of a level. Once the isolated parts of an area come together I also tend to make small in-engine changes on the geometry, polishing and fixing stuff - again with Mesh tools.

During all of this I also play around with the lighting a lot.

And that’s basically it. Of course before any of this I need a rough idea, what the challenges in an area are. I like to define those very loosely though and slowly fit them into the level as it comes together. Makes the design phase a bit easier - also lets you think about the level more logically - from what I know this is basically what Thief did - where a castle wasn’t just a bunch of hallways, but actually an environment where people could live in - you had kitchens, living rooms, servants rooms, bed rooms etc.
This is where the design principle clashes a bit with the King’s Field side, which was entirely focused on challenge when it came to level design. It had secrets where secrets technically didn’t make all that much sense and the entire ancient city doesn’t have a single bathroom. So there is a challenge in marrying those two approaches towards level design. It works out surprisingly well so far, but in doubt I’ll always go with the King’s Field approach, since interesting gameplay is still my primary focus. This is especially valid for secret walls, traps and puzzles.

Anyway, the alpha level is coming along really, really well. Here are a couple more WIP screenshots: