I have a few memories of watching this when I was really little. I remember it coming on tv a few times.
The movie starts off as a group of humans and maybe non-humans led by a human to this jagged mountain fortress that they have to get into to rescue something. One of the monster lookin dudes that was in the squad gets squished holding the trapdoor or some stone up so the others can enter. Pegasus may or may not be in the movie. At the end of the movie the mountain fortress collapses back into the earth.
Sorry it isn’t much to go on.