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Experts in this sort of thing (which we hasten does not include us), often suggest something called a 'difficulty curve' which in a perfect world should gradually arc up from a low-base allowing players to master the basics through an increasingly steep angle which will thoroughally test their new skills.
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Whilst everyone agrees it's important that a game offers some challenge so that a genuine satisfaction can be achieved from mastering it, exactly how much of a challenge to offer is rather less straightforward. The nature of 'difficulty' is something of a black-art in the world of games.
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