It didn't seem like Yuuka reached the tenth floor originally through this vertical shaft. Otherwise she wouldn't have underestimated it. It's possible the ghosts don't bother anyone coming down, only attacking those trying to climb up, but it's also possible they do, in which case Yuuka should have known about them if this was her route of choice originally. Quite honestly, though, I'm not sure why anyone would choose this route. It's looks pretty dodgy. Maybe they should have ascended using Yuuka's original route. Fetom might have known about it as well.