Ok, imagine you have a video game based on Them, the classic B-movie where giant ants attack America. Now, imagine you have a game based on War of the Worlds. Imagine you have a game based on Independence Day. Imagine you have a game based on Eight-Legged Freaks, or any other film with giant spiders in it. Imagine you have a game based on Godzilla.
Now, imagine that all these are blended into one game, and you have Global Defence Force, which plays like every giant monster film of the 50s rolled into one, but with all these overwhelming threats to life as we know it defeated by one lone bloke with a gun and a seemingly infinite amount of ammunition.
Sure, it's ridiculous, but it's also a hell of a lot of fun, and exposes an awful lot of games for the pedestrian, unimaginative, crawling things they are.
Plus, you can blow up buildings.