But -- don't forget it's a MacGuffin and so the details can change. Even substantially.
Like, should the characters actually be chasing after a person?
The place they are going through serves, or can serve, as a prison, but could be a strongroom as well. I could put some object down in there. All the more in that I have been a bit vague about why, exactly it's needed. It's important that the person who had the structure built was a lack-wit and entrusted building it to someone he should not have trusted. It's important that the MacGuffin be valuable to another person appearing in the plot. But neither of those require the MacGuffin to be a person.
