Idea fodder.
Modern scientists have discovered a deep cave centipede, the Hades centipede: livescience.com/51391-hades- ... vered.html
First, hey, there's an idea, a larger, or poisonous, or supernatural version of that centipede.
Second, the Velebit caves are apparently quite deep and extensive in places, and the Velebit mountains quite large. Quite a place for an oppidum, albeit somewhat close to Histria.