If you're old enough to remember 1st edition D&D, you might remember "Expedition to the Barrier Peaks". The premise is that your band of mideval adventurers stumble on this mountain cave with a bunch of strange monsters coming out of it. As brave adventurers, you go in to clean it out.
To the Players, this is a huge spaceship, but to the characters it's a strange cave labrynth.
James / nezeray