What a blast. Considering its time, place and intention Lewis did a good job with this. Tim Sullivan made a great point, in that it feels a lot like an old EC comic, in so many ways. I'm going to watch the rest of the Lewis films with that idea in mind.
You can see the basic structure of some later made horror here. The movie is aware that horror can come from characters enjoying inflicting the suffering more than anything else. Sado horror. This must have been a pretty effective shock in the 60's. The gore is tame now but still slightly disturbing as he makers knew that it was the imagination of the death scenes that really mattered. Again setting a standard for deaths in gory horror. The story is enough. It's no masterpiece. It's not as camp as Blood Feast (from memory: I could be wrong) but it's still camp. The bad guys are over acted and surreal. Funny when you're in the mood but not THAT funny. The setting is ridiculous but works. Twisted isolation. Maybe original? The music was good. I think you have to be in the right mindset to enjoy it nowadays. It kind of drags on.