Think about it: You're sitting around, minding your own business and all of a sudden unidentified alien spacecraft come shrieking down from the sky and in less time than it takes you to make a sandwich all resistance is utterly crushed and you're either dead or enslaved, never to see your friends or family ever again. In a competent film, this would be among the scariest of nightmare scenarios!
The utterly bizarre idea that the Psychlos hold of the "man-animals" being used as slave labor is laughable to them, when there are remnants of human civilization absolutely everywhere that people obviously built. Furthermore, if the Psychlos think humans are completely useless (as most of them do), then why keep them alive when they obviously have no qualms about killing them?
And why, if they were considered too stupid to be able to mine gold, were they shown in one scene working at a forge, which requires considerably more skill and training than mining?
An invasion force is, by its nature, more powerful than a force that is occupying a pacifed area. That's the only possible logical explanation as to how the man-animals pulled that off. As for the equpiment they used, there's just no excuse.
If they wanted gold, why the hell did they come to Earth, where it's so rare? It's like going to Mercury because you want to find water.
And if you wanted gold, and came to Earth even though it's so rare here, and had the planet for a thousand years, how in the name of Satan did you manage to never find Fort Knox? WE SAW THAT YOU HAVE GOLD DETECTORS, THAT WAS IN THE MOVIE.
Especially since the Psychlos were smart enough in the book to loot Fort Knox at the outset while somehow missing an armored car in downtown Denver, the source of the gold Johnnie uses in the book (which is better than the movie in a lot of ways, but still Made Of Suck). The Psychlos (understandably—they are aliens) mistake it for a tank, but strangely enough do not destroy this 'tank' during their initial attack.
Only one logical explanation possible, the tank was crippled for the psychlos, they probably know that there is nothing precious inside and maybe carry depleted uranium shell.
I can accept that a bunch of cavemen would be completely uneducated and their so-called history is full of Future Imperfect, but how far has mankind regressed in the last thousand years that Johnny and the others can't figure out what statues are in the beginning of the movie? Whatever this defect is, the learning machine wasn't able to fix it, because even after Johnny starts using it, he doesn't understand that markings on a map (such as borders) don't appear on the land itself.
Given that the Psychlo teleporter works by swapping two areas of space and that Terl dies by teleporting himself into what’s purported (at least on this page) to be a sun formed from his dead world, just what in the hell materialized on the Earth-side teleported?
SF Debris gave a rather convincing theory about the film. About midway through the film, Johnny loses his breathing tube while inside the Psychlo dome. It's likely that everything afterwards is actually a Dying Dream as the last trickles of oxygen reach his brain, effectively eliminating all the stupidity and plot holes in the second half.