@Kampfarsch The gameified rules guarantee that it can't be 'too hidden'. In the end, at best it can be equivalent to a really rare obscure collectible in a videogame... without a walk through. Think like needing to find 15 different Koroks in BOTW. While also not having the knowledge that there's Koroks til you find one.
So no, not impossible, just potentially very hard.
Also I don't see what's unfair about a riddle door? She could've used the same kinda thing, anyways. His rats probably wouldn't have been able to answer at all, too. Could've been a great decision. Like it said, the more complex the riddle is (however that's arbitrarily decided) the more it costs too. So he couldn't have actually made a riddle as hard as she thought it was.