The Terror Within feels like a made for TV movie [and I don't mean that as a complement]. This Roger Corman produced cheapie is predictable every step of the way. In fact, the audience is slightly ahead of the plot because they've seen much of it before in Ridley Scott's Alien (1979), including the classic "monster baby popping out of the stomach" scene that made alien famous. In Scott's hands, it's terrifying. In Corman's hands, it's boring and predictable.
In addition to massive stealing from another film, The Terror Within's monsters are erroneously labeled as "gargoyles." This is a complete misuse of the term and doesn't really fit the monsters who inhabit the film. They appear to be the product of a chemically induced plague that wiped out most of earth's inhabitants. The movie is never clear about the monsters' origins, leaving the viewer to ponder if they're from another planet or simply humans transformed by the plague.
Finally, the acting is pretty good with George Kennedy [Cool Hand Luke, The Naked Gun] getting top billing along with Andrew Stevens [The Fury, Dallas] playing second fiddle. The problem is that they've got very little material to work with so it never really lets them shine.
There are some good moments in The Terror Within but it never reaches anything that is compelling or original. It all feels like leftovers from the fridge and they're been sitting there for a while!
For more info check out the film's entry in IMDB.