This movie is not a literary dramatization of classical mythology. Instead it's a melodramatic action movie. The plot takes some long divergences away from the mythological sources, and has a few bits that don't quite make sense, but it does the job of carrying the characters from one action scene to another. The introduction tells us that Perseus is an honorary Son of Hercules, for no apparent reason except maybe to link it with the many Italian Hercules movies.
The acting is only occasionally good, but it's never terrible either. The costumes are pretty basic, but set the mood well. The sets are mostly simple too, but also get the idea across. There are plenty of extras in scenes that need them. The fights are sometimes well-choreographed and performed, but sometimes look dumb.
The lighting is almost always bright sunshine; even night scenes look sunlit, just slightly underexposed. The pan-and-scan was sloppy, and sometimes shows obvious losses, like people split vertically while they're talking. The cinematography probably looks better in widescreen versions.
The worst part was the monsters. The dragon looks decent, if low-budget, but doesn't move well in scenes it shares with actors. It's also a bit on the small side, but it's big enough to threaten a warrior in leather and bronze armor. It looked like a model of some sort, rather than a person in a dragon suit. In contrast to the dragon, Medusa is terrible, even though she's the title character in some of this movie's many titles. She looks like a leafless tree walking around on exposed roots, with a single glowing eye. Myths described her as a woman with snakes for hair, and looks so hideous her gaze turned people to stone.
The music sometimes took itself too seriously, but it kept the mood going pretty well. During the Medusa scenes, the score turned squeaky, as if the musicians were laughing at what they could see were dumbest scenes in the movie. Often it sounded better-suited to a Western than an ancient myth setting.
In parts where the movie is good, it's quite entertaining. When it's bad, it's still entertaining in a "so bad it's good" sense. As long as you don't set your expectations too high, you should be satisfactorily entertained.
