Creeper World by Knuckle Cracker is a game with a really interesting concept. The enemy is called Creeper and literally flows from Emitters. The Emitters ooze the Creeper which then behaves as a liquid flowing down hill, pooling or filling up holes, and spreading across the map. You would not want to take a bath in the Creeper because it is toxic. If it makes contact with your buildings then your buildings will be destroyed. Most buildings are destroyed instantly when touched by the Creeper, while the Blaster and Mortar can survive for bit. Your duel goal on each level is to protect Odin City (your home base and humanity's last hope) from the Creeper, and to make a connection from Odin City to the Totems. Once all the Totems are connected and powered up a rift will open which Odin City flies through and you win.
The nature of the Emitters means that very early in the game you will get a very good idea of what you are dealing with. That is to say that there are not really many surprises from the enemy. There can be some surprises from Emitters set to start at different points in the game and from Spores. Spores are airborne Creepers that come in waves. You can shoot them down with SAMs or wait for them to hit the ground and deal with the Creeper that are released. Successful Spore impacts do not create Emitters, just release a one time burst of Creeper. Few surprises does not mean it is boring.
Each level you play generally has a three basic phases. The first is to move Odin City to higher ground or a better tactical position if one is available. This will give you extra time to prepare for the flood of Creepers. The second is to mount a defense to keep Odin City safe. The third phase is to push back against the Creeper and connect to the Totems.
When building your base it is important that everything be connected to Odin City. While your military units store some ammo, without a connection and consequently no resupply, they can quickly run out and be rendered useless until that resupply. Civilian buildings stop working if the connection is broken. The connections are formed with Collectors and Relays. Collectors also create energy which is the lifeblood of your base needed to build and to power weapons. Relays are useful to bridge larger gaps but only if connecting to other Relays.
Collectors are limited in their energy collecting by area. Overlapping Collectors will not generate any extra energy. The area is represented by turning the area around them green. Some levels will require more energy then you can produce with Collectors. Reactors are more expensive to build then Collectors, but they produce energy without being limited by area. Storage increases your maximum energy level and Speed increases the rate at which the energy packets move along your network. Those energy packets are how you build (gray), ammo for your weapons (red), and how you charge the Totems (green). They will automatically travel the shortest path to their destination.
To defeat the Creepers you have a few weapons in your arsenal. The Blaster is your best defense against against an advancing Creeper. It kills a single layer with each shot. This means that it is not optimal for dealing with pools of Creeper. Blasters can 'cap' Emitters. Capping means clearing away the Creeper surrounding the Emitter and then the Blaster will kill the Creeper as it is emitted. Blasters are unable to fire at higher terrains. Mortars are your best way of dealing with pools of Creeper. A Mortar shot kills all layers of Creeper within the blast. Their fire rate is much slower then the Blaster and require more energy for each shot. They are not limited by terrain height. SAMs are your only way of protecting yourself against Spores. That is their only function. If the Spore hits the ground it is up to your other weapons to deal with the Creeper they produce. Drones are the equivalent of long range Mortars. They are also the only weapon that needs to be targeted manually, and the only weapon that can be targeted at all. Once you build your Drone it will arm and then will be ready to go on a bombing run. Once it runs out of bombs it will fly home and rearm ready to be redeployed. Like Mortars the Drone's bombs kill all levels of Creeper meaning that they should generally be sent against pools that are out of range of your Mortars. Blasters, Mortars and SAMs can all be moved. There is one more weapon, but it is only available in the last level or on custom maps. It is Thor and it is a super weapon. There is no strategy required with it. Once built just send it at the Creeper and it will obliterate everything in its path.
The game may sound simple because it is easy to understand, but the strategy and tactics involved can get complex. You need to strike the perfect balance. If you try to overbuild early on then you will waist time without enough energy. If you expand to fast you risk insufficient defense and being over run by the Creeper. Expand to slow and you may not have enough energy to mount a successful defense. Stretch your network to far without Speed upgrades and your weapons may be overrun waiting for their ammo resupply.
Creeper World comes with 55 maps, each of which has replayability. If you beat all the levels in the game you get the 'Double Down' option which doubles the Creepers in the Level. There is also a map editor for you to make your own levels and a place were you can review and download other people's maps.
At less then $10 Creeper World is worth the money. If you are on the fence about buying the game you can play a few levels online or download a trial version both for free. You can also create your own map with the map editor without buying the game, but can not play them until you have a full version. You can look at other peoples maps but again need the full version to play them. You can buy the full version for $9.95 and then instantly download it.
Video embedded below.
Update (3/19/2010): My custom maps.