Undead are spawning inside my caves!

Hi everyone!

After running 4 different worlds, I got some kind of a plan for an easy start, this includes having a cave for easy early stone and gold storage. My town usually is surrounded by stone fences, so goblins are not much of a problem, but the undeads keep spawning inside the caves / storage, I tried to light the area with the wall mounted lanterns but this did not work, the cave dimensions are 20 x 60.

Is is normal behavior or some bug?

Thanks!!

I moved 4 posts to an existing topic: [NaB] Monsters Spawned in mined out areas