I spent a fair bit of time, I believe, implementing "active" level elements; in short, elements of the level that move. My intention is that elements such as these help to convey that the environment is undead, rather than simply dead. Right now I have only one such: the ramp shown in the tweet above.
I've also created what I intend to be the final art for my two current enemy types, one of which should be visible in the screenshot shown first above.
A big addition is that of "triggers"--in short, simple collision-objects that, when entered, have some effect. Some spawn enemies and close associated doors; some control the difficulty-value, which determines which enemies may spawn; and a third type activates the previously-mentioned "active" level elements.
Less important is that I've changed the means by which enemies wander while unaware of the player: it's no longer based on randomly-generated waypoints, but rather a matter of steering randomly to the left or right while moving forwards, I believe. This has the advantage of not calling for any additional data to work reasonably well (such as knowing the location or dimensions of a given region), and I feel that it looks fairly good.
Finally, there was a miscellany of other, less salient-seeming changes. One that's perhaps worth mentioning is that I've reduced the player's speed; while still quick, and rather more so than the enemies, the player's speed feels to me to be less over-the-top than it previously was.
I may be forgetting a few things--I'm rather tired as I write this! ^^;
That's all for today--stay well, and thank you for reading! ^_^