Contrary to popular belief, this is the only one of the 5 "Ape" films that was deliberately written open-ended for a sequel. No sequel discussions for Planet of the Apes began until after the release. Beneath the Planet of the Apes was written to be the final entry. Conquest of the Planet of the Apes's original ending led to man's downfall without mercy. Battle for the Planet of the Apes was as deliberately planned as the last. After "Conquest", producer Arthur Jacobs was already considering the TV series; but "Conquest"'s profits were just good enough to justify another (this time) kid-friendly entry.
For the death scene, Corneilious and Zira were originally going to be ripped apart by a pack of Doberman Pinchers (the feared dog from that decade) by Doctor Hasslein. But producers thought the scene would be far too gruesome so they were killed by gunshots instead.
Zira's first line in the presence of humans, "Because I loathe bananas!", is a reference to Kim Hunter's real distaste for this fruit, which originated during the filming of the first Planet of the Apes. The actors portraying apes were required to keep their makeup on during breaks in order to save time, so the rest of the crew often called them monkeys and offered bananas to mock them.
Ricardo Montalban's character (Armando) gives a Francis of Assisi medal to Zira's newborn baby. Bradford Dillman who played Dr. Lewis Dixon in the movie, played Francis of Assisi in a movie back in 1961.
The trivia items below may give away important plot points.
Sal Mineo found the make-up uncomfortable, so the script was re-written to kill his character off earlier than planned. This was Mineo's final theatrical film.