By all accounts it's not a great film, with a meandering script, bizarre dialogue with virtually every line being sexual in nature, flat characterization, poor special effects, and a repetitive and unfitting disco soundtrack, but it's nevertheless entertaining as hell. It helps that the camerawork is surprisingly good for a film of this sort, and Jess Franco is clearly having fun in his role as director.
Special Effect Failure: Miguel's burns are clearly just sheets of melted plastic pasted onto the actor's face. Likewise, the boulder that falls from the ravine and nearly crushes Angela is obviously made of Styrofoam.