If there's one thing I'm good at, it's coming up with far too many explanations for something.
I'd try to telegraph the treacherousness of the cliff at the least, say by having piles of rubble at the base, have them see dirt fall while they're looking at the cliff, things like that. Illusions are definitely something that would come up somehow, but the cause may (or may not) come up until
after they've made the climb (and realized that there were illusions in place)
. By which I mean (since that was a clumsy sentence), the PCs may go into the area knowing there are fae or an illusionist in the area. Or they may not, and the cliff might be their introduction. It depends on a bunch of things.