While I don't believe Extraterrestrials had any part in building ancient monuments, I do believe that they shouldn't be categorized as paranormal. If the claims of Extraterrestrials are true (Alien Abduction, UFOs, Ancient Astronauts) they fit within the realm of science, they are explainable. If the claims of Ghosts are true, they fit outside of the realm of science, and are thus paranormal.