Despite dumb moments (especially Act One and the ending), the general idea behind the movie is fine. It's even moderately believable compared to a lot of horror films. Too bad there wasn't any actual exploration of the ruins. From the movie description I was hoping for essentially a horror version of Indiana Jones, or a more serious version of The Mummy. The Ruins is more inspired by The Evil Dead, The Cave, etc.
There are some weird moments which verge on being interesting, but this movie is largely boring and irrelevant. The characters are unlikable, the plot is haphazard, the script is irredeemable, and nothing is scary. Several scenes are comedic. For the first 45 minutes, this is merely a boring and poorly-done story of infidelity. One scene makes it seem like things will pick up, but Possession keeps going, meandering until the final 30 minutes.
The cinematography is good in spots and…
The best of the four. It's especially fantastic when you consider that it's based on a Disney ride. A ride, of all things. It's like someone saying Batman Begins was based on a roller coaster.