There are many different ways in which an escape room or event could be considered challenging, which is also why I stick to just considering puzzle difficulty rather than overall room difficulty on my review blog.
What constitutes an enjoyable or meaningful challenge is probably something that each individual gamer will have to decide for themselves (if at all), but here’s my own entirely biased take on things. Roughly in increasing order of personal preference, here’s a list of different ways in which a room might be challenging. (I’ve left out “bad/ambiguous/illogical puzzles”, since I’m not sure anyone actually sets out to do this.)
– Simple mismatch between a reasonable expected solving time and the actual time allotted. Simply giving 45 minutes instead of an hour could make all the difference between a tough but fair room, and an unreasonably difficult one.
– Tedious puzzles. One escape room company in Singapore is (was?) notorious for having puzzles that required large amounts of counting or visual searching. There’s no thrill in such drudgery. This can also be a problem when puzzles aren’t innately tedious but “go on too long” — for instance, having to play a sequence of 20 musical notes instead of 10. It doesn’t take additional skill to do so, just more time.
– Hard logistical constraints. Darkness is a common one, especially when combined with a limited number of torchlights. I personally find this a cheap way to increase the difficulty. Dim lighting is fair if you want to obscure the existence of a clue or create an atmosphere, but it shouldn’t be used to make up for puzzles that are too few or too easy.
– Red herrings. I think rooms shouldn’t be so bare that clues and puzzles become obvious, but neither do I think rooms should have red herrings which are too numerous or simply unfair (those which match an existing puzzle, say).
– Memory work. I can see the argument for this being a valid skill to test, but it just doesn’t improve the experience for me.
– Computational work and complex logic puzzles. I can solve simultaneous equations, a sudoku, or a logic puzzle in my own time, were I so inclined; if I’m in an escape room, I’d appreciate more relevant, site-specific puzzles.
– A strong need for teamwork, including co-operation, communication, and human resource allocation. Split-start rooms are a good example, as well as events which might require a team to split up. Not only can this liven up the room experience, it also allows for clever or inventive puzzles. Most split-start rooms don’t stay that way for long, though, which I think is a good idea.
– Requiring physical precision or dexterity. This can be a particularly fun or refreshing element to have, especially after the first generation of escape rooms which were more lock-and-paper affairs. I do appreciate rooms which require a bit of this, but like everything else, it can be taken too far. Although they’re called escape rooms, I think the genre is generally thought of as highly reliant on puzzles and problem-solving, rather than mere skill in carrying out physical tasks.
– Need for searching and/or keen observation. Having clues that are just hidden enough can improve the experience of an escape room, compared to having the puzzles obviously laid out for you. Of course, there’s still a difference between a room that requires tedious searching (e.g. several shelves of books, all of which need to be checked; lots and lots of drawers) and one with clever hiding places (e.g. a key stuck to the underside of a drawer).
– Intuitive leaps. I personally feel that the core of a good puzzle is the central intuitive leap, or the aha, as it’s commonly known in the puzzle hunt tradition. Everything else after that is just some variation on execution, even if it’s non-trivial. A good aha might be particularly clever, apt, or creative; it also has to be fair.