by Hermjard
A game is fun, if all of it's participants can (at least theoretically) come into knowledge of the complete rules, which are equally binding for everyone in advance, instead of a matter of negotiation during the game.And a game is challenging, when I have to admit, that my opponent made better use of the rule set than me, and won because of that.
There is a requirement for that though: you can only make best use of the rules and plan secretly, if you have a clear image about the rules (and the same image than your opponents), without need to ask your opponent about them during the game.