AERODYNA -- I'd say this level is fairly hard, and (barring extreme luck or somebody really skilled at controlled falling via the jetpack) the player will enter the doorway already dead. (I don't know if you're changing that in the new version or not).

FORTKNOX -- I'd peg this level at medium difficulty. Several times it takes advantage of the brief invincibility that comes from entering a teleporter (I hope this hasn't been removed in the Retrofit). I'm not overly fond of the ladder room with the trackbot -- I think it works, but I just have the feeling it could be better somehow.

LADDER1 -- A focus on jumping and ladders...medium, medium-hard difficulty, I'd say. Aesthetically, I'm not too fond of this level, since I think having spikes just hang in midair seems so ugly. I tried to clean this up some in this and a few other levels, but I wasn't 100% successful.

LADDER2 -- More jumping and ladders, but this time with barrier-switching fun. Easy-medium.

LADDER3 -- Another level which uses teleporter invulnerability. A lot of long jumps in this one! The red barrier in front of the door actually used to be under the door -- I changed this out after I realized that turning the barrier off made the door invisible. Hard difficulty.

OBLIVION -- This level is, well, bastard hard. I've cursed at it more than once, and I designed the dang thing. It's theoretically completable, as I've finished each individual section at least once. Detriments: It has the same 'ugly spikes' syndrome I complained about in LADDER1 (The row of fake stone blocks near the top is only there to give the spikes above them 'support'), and it's not technically a spiral (since it changes direction at the last minute).

Fun anecdote: I made all these levels (and about 20 others) on my laptop years ago, which I accidentally killed around the time of my high school graduation (2002). For years it just sat in the corner gathering dust until one day I managed to take out the hard drive and plug it into my new computer -- it lived just long enough to salvage the levels I had made so many years prior. Hooray!