Additionally, King Caesar's battle with Mechagodzilla features the music piece of Godzilla vs. Kumonga from the film, Son of Godzilla. This comes as no surprise since the composer of this film is also composed the music from the latter film as well.
What Could Have Been: The original idea for the film called for Godzilla to team up with Anguirus and Mothra to fight against a group of aliens known as the "Garugans". The idea was eventually rewritten and both Mothra and the Garugans were replaced by King Caesar and the Black Hole Aliens (and Mechagodzilla) respectively.
Anguirus suddenly gaining the ability to burrow was not an accident. Originally, Baragon was to play the role of the monster that Mechagodzilla (while disguised as the real Godzilla) beat down, but he was snubbed, likely because, as with in Destroy All Monsters, his suit was not fit for use, while Anguirus' was still in good condition for a suit built in 1968.