Commander Spleen: Thanks, I'll take a look at those editors! Monotone does seem a little overpowered for what I'm trying to do. As a side note, the game does render in good old 320x200 and then upscales to the nearest integer multiple that fits the browser window

One of the physics differences compared to keen is that player movement is updated at 60Hz while CK IIRC uses a much lower tick rate? I think Keens movement feels a little heavier and chunkier than Rixots which is something I'd like to try to replicate better. I tuned the current movement code by just playing both games side by side and making adjustments by feel.
Roobar: I agree, forced replaying is not much fun. I also feel like the early levels should be a little easier to better introduce you to the controls. There's also some trial-and-error type deaths in several maps that I'd like to fix. At least in the early levels the most dangerous places should be optional challenges and be clearly indicated as such. An overworld map would help here as well, I like how in CK many of the levels are optional and can be skipped if you don't feel like replaying them.