I have done levels 1 and 28.
I have done level A.17 (and its dependencies), the mechanic used by this level, and some art for the Theory A area.
I have done Level 4 (just a simple introduction to a mechanic). I swear I'm not involved in the creation of Captain Clement the "inclement"! ... at least, not directly, I guess.
I've done Level 22 and some strange lore (Level 34)