Not even whether it's a person or an object.
If it's a person, he's going to have to die, but since the climax of the story turns on their being attacked by another character who doesn't want them to get what they are after (whatever it is) it does not, in fact, matter very much. The conflict is breaking into to get the MacGuffin and discovering why the route was made so hard -- not the reasons that were professed when the location was built. All of which dregs up the main character's carefully preserved secrets.
Still, it might be nice to decide what it is. It might have some effect.
Hmmm, maybe the characters don't properly know either. That sounds like something the original idiot would omit to tell anyone.