Yeah, cavern spaces are clear. I guess what that means is that they cost 1 movement per space and that they doesn't block anything (except the walls that is.)
The monument would make that cost 2. (And I assume it's got a range, I don't know whatever that uses the name aura or not but I assume the range is measured as for auras regardless, as in not through cavern walls but travel into caverns.)
I agree completely.
Obviously, I'm just guessing based on my understanding of the rules, but I think you are quite correct.