Creeper World is a game that can be found in several forms; among these are online versions and a downloadable demo, and also a full version that can be purchased for about twenty dollars.
The crux of the game is that you have to fend off something called the Creeper. The Creeper is an amorphous purple something that is supposedly all but unstoppable and has wiped nearly all of humanity (on thousands or millions of worlds) out. In practice, it's far less terrifying. In fact, it's extremely similar to the "Smooze" from the My Little Ponies movie (I watched that with my sister once, and remember it considerably better than she does-it's probably the trauma), except that instead of driving it back with a bunch of magic flying horses, it's driven back by a massive series of fortifications and weapons systems that you must build.
Why do I make this comparison? Because the Smooze was hyped up in an extensive and annoying musical number as being unstoppable, and there's a rather extensive intro for the Creeper that plays it up similarly... and it just isn't true.
As a matter of fact, I've discovered that it's often rather easy to hold the Creeper back indefinitely. There are several key features of the game that impose difficulty on you, and there are several key features that let you fight off the Creeper with great efficacy.
First off, you need to control as much area as possible. The green area on the map represents the places that you have energy producing Collectors in; if you cluster them tightly, they're less efficient.
Second, on later maps (in the demo, you're introduced gradually to each new feature-I'm assuming that it essentially works simply as the earliest part of the full game) you must build SAMs, which are used to hold off Creeper spores, which periodically rain down and devastate areas they hit by filling those spots with Creeper.
Third, if Creeper reaches and touches one of your buildings, it starts very rapidly draining its health, sometimes taking it out instantaneously.
Fourth, if the Creeper has the high ground, your most efficient weapon, the Blaster, doesn't work on it.
However, that's just about all the bad news there is.
Taking territory from the Creeper is extremely easy if you have the high ground. It's harder without it, but not impossible.
The spores are less of a threat than they're made out to be-they're barely even a nuisance. They always come from the same direction, and you can dynamically place your weapon turrets, so you can just push forward with your SAMs after you get enough to protect your front line, and they'll cover your entire base.
There's no real defense against the Creeper killing your buildings if it touches them, but you can move your city (the big part of that network) in an emergency, so it's harder to be trapped than one might think. The Creeper moves like water, flowing downhill and filling in low-lying areas first, so it's very predictable, however. It also helps to separate the Creeper sources (those black circles deep in Creeper territory) from each other.
And finally, while the Blaster can't take the high ground, Mortars and Drones can, and they're far more potent per attack than the Blaster. The Mortar is basically the butter part of your bread and butter offensive, while the Drone is like a reusable nuke. (No pictures of the Drone, I'm afraid-I wasn't playing the demo level that had it.
One of the limitations of the basebuilding and energy supply of your base is that it happens in the form of "packets" that travel along the base's connection lines, and these packets have a definite, finite speed. There's a way you can improve this speed-constructing these red rhombus-shaped structures. It can really make an astounding difference...
...especially when you build a lot of them.
Initial inspection made me think that this game was a clone of The Space Game, but it actually has at least as much depth as the other game, and a more interesting enemy and game balance. According to the main page for the game, it also has something like 3.6 million game maps in the full version, so I'd say it has high replayability, too. I can definitely recommend playing the demo version, and if you decide you like it, I suspect that you could be playing the full version for the rest of your life and never run out of new maps to play.