Even if Leonard Maltin rates this higher than KING KONG ESCAPES (1967) giving it ** against the latter's BOMB I seemed to enjoy that one more; mind you, both are essentially dreadful
but, at least, the KONG film had a good deal of campiness going for it whereas this is merely dull (in spite of megalomaniac aliens and a plethora of monsters)!
In a way, the film is a rehash of Honda's own MONSTER ZERO (1965), which I watched some time ago where you also had an interplanetary attempt at world domination that saw the involvement of a clutch of Toho Studios monsters. Here, however, we get a greater variety of the latter: dinosaurs Anguirus, Baragon, Gorosaurus, Godzilla (along with its annoying baby offspring Minilla) and the flying Rodan, the spider Kumonga, the snake-like Manda and the worm Mothra. Though all these are eventually brainwashed by the aliens to do their evil bidding, destroying several of the world's leading cities in the process, the latter provide their own monster King Ghidorah for the grand finale (during which it takes on all comers but, obviously, emerges the loser). That said, only Godzilla Snr. and Rodan receive any decent exposure throughout the film!
While it is accompanied by a typically rousing score, the action often comes across as confusing rather than exciting (not to mention unintentionally hilarious during the climactic monster mash). As for the plot, it's all quite dreary and lazy: all the Earth monsters have conveniently been assembled on an island to be studied (yeah, right just try to get a blood sample from Godzilla, or Rodan, for that matter)...but, then, the aliens (naturally possessing superior intelligence and frightening assurance they must have missed MONSTER ZERO, for they're deluded into thinking that the monster ensemble can't hold a candle to King Ghidorah!) manage to fit all of them with a conditioning device behind the ears!!
Finally perhaps its most amusing elements to me were the fact that the island personnel (channeling the monsters' individual movements on a monitor) are able to judiciously cross-cut, for suspense purposes, between attacker and prey (a moving train) during a demonstration; the film-makers could have fitted the aliens with make-up or, at least, a mask so as to hide their all-too-obvious Asian countenance (there's no suggestion in the script that that they may have adopted such a disguise in order to go undetected among the Earthlings); also, for a kiddie film, a lot of the human characters are heard cursing the aliens (sure, it's in Japanese, but the implication is unmistakable).