(4) Execution levels where it's obvious what to do (and you usually get just the skills you need) but doing it is deliberately tricky, like The Crossroads. These seem to be unpopular, but I like them so long as they're not too annoying, because I like the adrenaline rush and feeling of elation when you get it right. Bomber timing is an obvious sub-type.
(5) Classic hybrid type -- the overall approach is clear but getting past individual obstacles may be a puzzle; you have enough spare skills to enable different approaches, but clumsy execution or attempting to brute-force a solution may run out of essential skills, usually builders; execution is often difficult as well. Most of the Taxing and Mayhem levels are of this type, and I think this type is my favourite, though I do like the feeling of being baffled and then getting the solution to a pure puzzle level, as well.
(6) Misdirection levels -- pretends to be a type 5 but "normal" solution is impossible. Often the intended solution involves a circuitous route that looks worse but turns out to require fewer skills. I can't think of an example from DOS Lemmings, but Poor Wee Creatures would be one if it gave you fewer builders. There are many of these levels in the harder categories of the Genesis game.
(7) One-offs that don't fall easily into any category, often joke levels. See All or Nothing and No Problemming.
