Creeper World is technically a tower defence game, but unlike any other you've played before.
In a normal tower defence game (and even abnormal awesome ones like Defender's Quest
) you'll have enemies coming towards a point or object you're defending and you're tasked with fending them off by placing towers either along a pre-defined path or by creating that path using walls or the towers themselves (this is known as 'mazing').
In Creeper World there is only one enemy, the Creeper. The Creeper is a fluid
of alien origin and malicious intent, it absorbs and destroys anything constructed it touches. This changes the entire dynamic and feel of the game.
The goal (in most levels) is to activate totems that are strewn about the (bird's eye perspective 2D) map without having your base be destroyed by creeper. Creeper flows in continuously from spawn points and you build out a network of structures from your command ship to fend off the creeper (or even destroy its spawn points). The buildings are either combat focused (the traditional tower defence type towers) or utilitarian.
Because the resource system is quite different from a regular tower defence game as well (and is surprisingly similar to Supreme Commander / Total Annihilation instead), you have 'Energy' which is used to build structures, arm your towers and charge up totems. Energy is stored in your command ship and then flows along the nodes formed by the buildings you created, like a spider web that slowly grows outward (and not unlike the creeper that slowly washes over the map).
A simple level progression would be to land your command ship at a strategically good location (high ground, remember that your enemy is basically a liquid so it'll flow into the low spaces first), start building collectors (buildings that gather energy from their direct surroundings) and laser towers to protect the collectors which you slowly move up with the front as the collector field grows in size (all offensive towers can be moved across the map whenever you want).
Of course things get far more complicated later on, with the addition of new towers including mortars, bombers and snipers. New enemies like a creeper-formed network that will always transmit the creeper even if it's uphill, building-disabling enemies living on this network and even aerial creeper that will occasionally spawn from specific enemy buildings and head for your base.
All these things (and a lot more) are slowly introduced over the course of an excellent 30 or so campaign that even has a pretty engaging storyline (the game is set thousands of years after Creeper World 1 and 2, so you don't need to know all that much from them to get the gist of things. Unfortunately they were never released on Steam. For a sample of the gameplay from Creeper World 1 and 2 (and even 3, in case you want to try it out) there are in-browser options available here
There is one mission right at the end of one of the two branches of the game's plot that is very different from the others though, and not in a good way. Unlike pretty much the rest of the game it has a time limit, and a harsh one at that. In the rest of the game (especially within the campaign) it's pretty hard to lose as long as you take things slow and don't over-extend yourself. This one mission completely messes that up and asks you to rush in and beat the clock. It's not a bad concept I guess, but terribly executed and I ended up downloading an in-mission savegame in order to bypass it because I just couldn't beat it.
Other than that single (but major) gripe there is nothing but praise for this game. The campaign mode (which will last you a good long while) is only the tip of ice berg, as there are thousands upon thousands of additional levels that you will need a lifetime to complete. These range from fun levels the beta testers came up with, to nasty challenge levels the creator put in, to a whole universe of bonus levels, to procedurally generated levels. There is also an in-game level editor, complete with a way to directly share them with others and play their levels and tools to change anything in the game.
Go give the game's free version a shot, and if the gameplay at all grabs you this is an excellent value purchase. Just remember that there's one super annoying mission in the campaign mode.