They moved into a werewolf habitat
Unfortunately the people who settled that Fort that's where they set up shop. The problem was already out of hand before the sisters arrived. The sisters coming into the fort actually made little difference as there were already infected people (the boy) in it going to bite someone and infect them.
The film does a great job at creating an atmosphere of paranoia and threat. Basically as soon as the gate opens at the end... Well that's it.
What a place to set up camp.