In truth, this commandment is actually very closely related to the commandment about Learning. This is because if there are no gameplay paths that the player can come up with that give a reasonable chance of victory, or if the player can choose an arbitrary path (i.e if the game is too easy or too hard), then the player either cannot or does not need to learn anything! If we accept the premise that learning is a huge aspect of the fun of strategy games, then providing the correct challenge level is absolutely critical in creating an enjoyable experience.
Games have tried many methods of tailoring the challenge level correctly over the years, some overtly, others more subtly. One classic example is the concept of “grinding” in early RPG systems like Final Fantasy. The ability to grind (gaining power through some repetitive action) ensures that players of any skill level can eventually surpass whatever challenge the game throws at them. In theory this allows players to correctly tailor the challenge level to themselves. In practice it often results in players grinding until the game presents no strategic challenge (as players will often use the simplest solution available, even if it is less fun for them!). Not only that, but grindingly inherently involves engaging in a low-value activity in exchange for power, which means part of your game system is inherently boring to engage with (setting aside the quirk of human nature which assign some base-line satisfaction with gaining quantifiable power, as evidenced by clicker games).
Another common method of scaling difficulty is to allow players to simply select the difficulty from a menu at the start of the game. This is problematic as the player has no way of knowing which difficulty level will be appropriate for them in your game system until they have engaged with it. A more modern “fix” is to allow players to re-select their difficulty at any time from a menu. The problem with this approach is that it again gives players a trivial way to bypass the challenge and learning of your game, and therefore a lot of the fun! Additionally, I think that part of the job of a game designer is to do upfront work to craft a game experience the player will find enjoyable, and to some extent that includes selecting the difficulty. Players shouldn’t have to do the designer’s work for them (although I will admit that at times this is not feasible, in which case practical concessions need to be made).
Am I more of a “Ultra-Violence” guy or a “Nightmare!” guy…
BrainGoodGames have taken another approach, as outlined in the article on Learning. They use a single-player ladder system to develop a sense of player’s skill, and then adapt dynamically to continually modify the challenge to be suitable for them! In my estimation this solves a lot of the fun-circumvention problems of other systems, and removes some design burden from players. Win-win! A further augmentation present in Militia, and likely soon to be included in other BrainGoodGames is the inclusion of a “Placement Match” system to allow players who feel the difficulty is incorrectly calibrated to quickly set it to a (ideally) closer challenge level. This also allows experienced strategy game players to skip ranks that are too easy for them, and players’ to opt in to re-calibration after a large balance patch!