The Cyclops (1957)

Starring James Craig, Gloria Talbott, Lon Chaney, Jr., Tom Drake, Duncan Parkin
Directed by Bert I. Gordon
(actor & director credits courtesy IMDB.com)

A young woman charters a plane to find her missing fiancee, who crashed three years before in a forbidden Mexican valley, home to immense creatures and a 25-foot one-eyed giant.

One of several science-fiction pictures made by Bert I. Gordon that he also produced the special effects for, the film's pretty short on story and character development, but includes some decent matinee thrills, featuring a fearsome makeup for The Cyclops, and solid, if not spectacular, rear-projection sequences.  Although the actors are largely playing nothing more than stereotypes, they're an engaging lot, particularly Lon Chaney, Jr. as a greedy co-financer of the expedition.  I found it to be a good bit of diverting fun.