To start? Barricading the main entrances to the hotel, for one. Heavy furniture is a thing. You know some people with muscles, right? My muscles is apparently going to go be an idiot, so I'm not counting on him. Entrances include windows. Having people on the first floor barricade their windows. I don't know if it can climb and jump, but it can't hurt for everyone to secure their windows on all floors. Lock off the rooms with large windows, like the dining room and ballroom, but bring the tables and chairs from the dining room to help with the barricade. Limit access to the basement levels and ground floor. Alert the people who are supposed to be doing patrols to, I don't know, do patrols or something. Ask anyone who can communicate with ghosts (yes, this includes you, Klaus) if they can hear what they're saying about all this. At least we'll know if they're part of this or are in just as much of a shocked state as everyone else.