Spirit wards? Not sure that'll work here like you're used to, but it could be worth a try.

[Consider him doubtful, but he doesn't let his cynicism slip through in his voice too much. Instead, he nods, encouraging. Hell, it isn't like he knows any better than anyone else here what works and what won't, he's just not used to all this magic stuff some people seem to have. He'll keep his pistol at the ready.]

We can't really make much use of the usual things like lookout towers with all this darkness, but trenches and walls and traps might still be helpful.