Arnold Schwarzenegger was originally to have reprised the role of Conan in this film, as the Red Sonja character was a spinoff of the original Conan novels. Ultimately, a new character was created who was essentially Conan in everything but name.
The original theatrical trailer of this movie contains cutting room-floor footage of scenes glimpsed only briefly in the film's prologue: Sonja, left to perish in her burning home, is saved by her dying brother Barlok; Gedren, prior to her disfigurement, addresses her soldiers and slaves.
The name of 'Ronald Lacey (I)''s character, "Ikol," spelled backwards is "Loki." Loki, in Nordic mythology, was the God of Evil and Corruption, infamous for being a jealousy-prone troublemaker.
Sandahl Bergman was initially cast as the title character, but decided on portraying "Queen Gedren" instead, to help avoid typecasting.
It is commonly believed that Robert E. Howard created the character of Red Sonja in one of his Conan short stories. In reality, he created a character with a similar name (Red Sonya) that appeared in a minor role in a non-fantasy work of historical fiction. "Red Sonja", the character popularized by the 1985 movie, actually first appeared in a Conan comic book written by Roy Thomas and illustrated by Barry Windsor-Smith.
Arnold Schwarzenegger himself eschews this movie as one of his worst, yet he (laughingly) claims that it's an excellent disciplinary tool for his children: "I tell them, if they get on my bad side, they'll be forced to watch ‘Red Sonja’ ten times in a row. Consequently, none of my kids has ever given me much trouble."
Maria Shriver said in a TV interview that at the premiere of 'Red Sonja' she remarked to her husband, "If this doesn't kill your career, nothing will".